Martial artists are more invulnerable than you think. Ranma can survive both being frozen solid and having his skin heated to the melting point of iron; are you so sure cyanide's going to bother him that much? Even if it does, how do you deliver it? They don't all gather in a nice little group for you: do you slaughter all of Tokyo to take out its martial artists? Even if you do, what about the ones that inevitably do survive due to not being around or not drinking the water before their family did?
Military technology is relatively impotent against the kind of threat somebody like Ranma poses. He can laugh at guns, you can't bomb a single person (trying to get rid of single specific people in the real world is often hard enough), and they all look exactly like you. There's no way to tell who's a demigod and who's a regular soldier until suddenly one guy in your headquarters flips out and kills everybody.
Should you win despite this, then who fights the gods and demons and remaining rogues? That's a far bigger problem than martial artists worrying about food, since they're a small population of excellent hunter/gatherers.
I won't argue that destroying civilisation would certainly inconvienence martial artists, but not nearly as much as trying to kill every Kunou would to said civilisation. Even if the "norms" won, it would be an utterly pyrrhic victory and new martial artists could arise at any time.
Martial artists are not immune to repurcussions from normal authority for their actions in a technical sense, but in all realistic sense they are: you can't crack down on them with ordinary means, and using extraordinary means like military force is to invite a war that will cost you far more than letting them do what they want. Plus, martial artists and such tend to be heroes to the public - when one of them's out of control, other martial artists handle the problem, not the SDF, and they handle it better than the SDF could.
In the land of the blind, the one-eyed man is king. The organisation of society in a world where some people are demigods that answer to noone but themselves is that the people with the real power also gather as much secular power as they please. So yeah, the unspoken convention is perfectly plausible - indeed, it seems to be what is the case.
"Interfering in someone else's challenge is just not done"? Pshaw. Every main character in Ranma 1/2 does it multiple times. They involve themselves in whatever they feel like and justify it however they wish. Besides, Jadeite threatened to destroy the city. Why would anybody leave their imminent demise up to anyone else? You're saying that if Ranma sees some pretty boy goob ranting about how he'll set the city on fire if his demands aren't met, Ranma will go home and go to bed because the pretty boy was challenging someone else? Not likely.
Off-hand note: the Orochi isn't really a god; it's a legendary monster, like the Hydra. The question of it having multiple avatars (spoiler warning) was addressed in the Ryugenzawa portion of the fic, where Link specifically watched for the reaction of the high priest of the Orochi to seeing the Orochi from Ranma. It has nothing to do with him at all and he dismissed it, which confirms what she suspected: it's a different Orochi. The thing in Ryugenzawa is completely incompatible and has no connection to the thing which wants to incarnate in Chris, which has no connection to the mitamas and Murakumo. Really, it would be unbelievable if this were not the case: the backstories of King of Fighters and Blue Seed are two completely different histories with completely different takes on the Orochi that have no real connection to each other despite being based on the same myth. The Orochi in Ranma is also completely different. But it was still referenced, and observed by Link (as well as little-Chris), how the three different Orochis had nothing to do with each other.
Arkanphel doesn't date nearly as far back as you think, either. He was created shortly before the last Ice Age (which he inadvertantly caused). And the background with Uranus seeding all life on Earth doesn't at all gel with the background of the Silver Millennium and the larger background of the Sailor Wars unless you start kludging considerably and ignoring the fact that in Sailor Moon there's huge amounts of magical items and beings swarming around the galaxy which Arkanphel has never noticed or reacted to in over seven centuries of being awake and active. Plus, of course, Arkanphel and the Zoalords never respond to anything that happens in Sailor Moon, and Sailor Moon (despite her capital-D Destiny) is not told nor hinted nor in any way forced to deal with the fact that she was in a world controlled by Chronos in all but name, where every government and military and police force and media outlet is controlled by monsters loyal to Chronos, who would most certainly be interested in finding out more about a group of magical vigilantes.
None of this is outside Ranma's sphere of interest, either. Wherever did you get the idea he only fights martial artists? He's been tapped to deal with rogue gods and escaped demons and Genma specifically says in the latter case it is a martial artist's responsibility to deal with this sort of thing.
The more series' you involve, the more heavily you have to start asking people to suspend their disbelief and the more kludgy and unbelievable explanations inevitably get. Very few series' can even smoothly crossover with one other series in the same genre without conflicts that require tricky explanations. For instance, a crossover between Ranma and Street Fighter would seem simple, right? Not so fast! Ignoring the many backstory elements in Street Fighter that really ought to come into conflict with Ranma in the year he's in Nerima (in which he and his friends became very well-known), you run up against the harsh fact that in Ranma 1/2, no human being can throw chi blasts. They have sort of projectiles, but they're balls of air held together by emotion chi (and can't go very far), or vacuum blades that require immense powering up, or are relatively small attacks used by old masters of great power (Cologne never uses a "ki blast"; the best she can apparently do is a very short-range compressed air attack; Happousai's got more raw power than that, but only seems to use it in his giant form, which exhausts him). The only characters in Ranma who can use real chi blasts in the Ryu's fireball sense are Miss Hinako, who was mutated to be able to do it, and Herb, who is stated to have an inhuman mastery of chi. But in Street Fighters, chi blasts are so easy to do that people can pick them up with practically no effort, even total losers (like Sean and Dan) or people with no training beyond watching videos (Sakura). It strains credulity to the utmost to say they're working on the same assumptions; how can Herb be more powerful in chi than any human (specifically stated) when his best stuff is no more impressive than Ryu's super attacks?
Of course, this can explained away. Theoretically, any incompatibilities can be. But it's hard to do it smoothly and believably, and becomes exponentially more hard as you add more series'. Eventually the weight of the conspiracies and "Oh, well, these old masters just didn't know anything about these techniques that dozens of people in this series regard as commonplace" and the other kludging adds up.
That being said, few people other than Link think the world is artificial or somehow unreal. Most people probably assume there was a conspiracy, or forces moving that nobody knows about just yet that kept the delicate balance of the world until they abruptly broke down.
As for the time travel device (you mean the Black Moon?), it's an interesting question indeed, which I am unwilling to answer due to spoilers at this time. ;p
Military technology is relatively impotent against the kind of threat somebody like Ranma poses. He can laugh at guns, you can't bomb a single person (trying to get rid of single specific people in the real world is often hard enough), and they all look exactly like you. There's no way to tell who's a demigod and who's a regular soldier until suddenly one guy in your headquarters flips out and kills everybody.
Should you win despite this, then who fights the gods and demons and remaining rogues? That's a far bigger problem than martial artists worrying about food, since they're a small population of excellent hunter/gatherers.
I won't argue that destroying civilisation would certainly inconvienence martial artists, but not nearly as much as trying to kill every Kunou would to said civilisation. Even if the "norms" won, it would be an utterly pyrrhic victory and new martial artists could arise at any time.
Martial artists are not immune to repurcussions from normal authority for their actions in a technical sense, but in all realistic sense they are: you can't crack down on them with ordinary means, and using extraordinary means like military force is to invite a war that will cost you far more than letting them do what they want. Plus, martial artists and such tend to be heroes to the public - when one of them's out of control, other martial artists handle the problem, not the SDF, and they handle it better than the SDF could.
In the land of the blind, the one-eyed man is king. The organisation of society in a world where some people are demigods that answer to noone but themselves is that the people with the real power also gather as much secular power as they please. So yeah, the unspoken convention is perfectly plausible - indeed, it seems to be what is the case.
"Interfering in someone else's challenge is just not done"? Pshaw. Every main character in Ranma 1/2 does it multiple times. They involve themselves in whatever they feel like and justify it however they wish. Besides, Jadeite threatened to destroy the city. Why would anybody leave their imminent demise up to anyone else? You're saying that if Ranma sees some pretty boy goob ranting about how he'll set the city on fire if his demands aren't met, Ranma will go home and go to bed because the pretty boy was challenging someone else? Not likely.
Off-hand note: the Orochi isn't really a god; it's a legendary monster, like the Hydra. The question of it having multiple avatars (spoiler warning) was addressed in the Ryugenzawa portion of the fic, where Link specifically watched for the reaction of the high priest of the Orochi to seeing the Orochi from Ranma. It has nothing to do with him at all and he dismissed it, which confirms what she suspected: it's a different Orochi. The thing in Ryugenzawa is completely incompatible and has no connection to the thing which wants to incarnate in Chris, which has no connection to the mitamas and Murakumo. Really, it would be unbelievable if this were not the case: the backstories of King of Fighters and Blue Seed are two completely different histories with completely different takes on the Orochi that have no real connection to each other despite being based on the same myth. The Orochi in Ranma is also completely different. But it was still referenced, and observed by Link (as well as little-Chris), how the three different Orochis had nothing to do with each other.
Arkanphel doesn't date nearly as far back as you think, either. He was created shortly before the last Ice Age (which he inadvertantly caused). And the background with Uranus seeding all life on Earth doesn't at all gel with the background of the Silver Millennium and the larger background of the Sailor Wars unless you start kludging considerably and ignoring the fact that in Sailor Moon there's huge amounts of magical items and beings swarming around the galaxy which Arkanphel has never noticed or reacted to in over seven centuries of being awake and active. Plus, of course, Arkanphel and the Zoalords never respond to anything that happens in Sailor Moon, and Sailor Moon (despite her capital-D Destiny) is not told nor hinted nor in any way forced to deal with the fact that she was in a world controlled by Chronos in all but name, where every government and military and police force and media outlet is controlled by monsters loyal to Chronos, who would most certainly be interested in finding out more about a group of magical vigilantes.
None of this is outside Ranma's sphere of interest, either. Wherever did you get the idea he only fights martial artists? He's been tapped to deal with rogue gods and escaped demons and Genma specifically says in the latter case it is a martial artist's responsibility to deal with this sort of thing.
The more series' you involve, the more heavily you have to start asking people to suspend their disbelief and the more kludgy and unbelievable explanations inevitably get. Very few series' can even smoothly crossover with one other series in the same genre without conflicts that require tricky explanations. For instance, a crossover between Ranma and Street Fighter would seem simple, right? Not so fast! Ignoring the many backstory elements in Street Fighter that really ought to come into conflict with Ranma in the year he's in Nerima (in which he and his friends became very well-known), you run up against the harsh fact that in Ranma 1/2, no human being can throw chi blasts. They have sort of projectiles, but they're balls of air held together by emotion chi (and can't go very far), or vacuum blades that require immense powering up, or are relatively small attacks used by old masters of great power (Cologne never uses a "ki blast"; the best she can apparently do is a very short-range compressed air attack; Happousai's got more raw power than that, but only seems to use it in his giant form, which exhausts him). The only characters in Ranma who can use real chi blasts in the Ryu's fireball sense are Miss Hinako, who was mutated to be able to do it, and Herb, who is stated to have an inhuman mastery of chi. But in Street Fighters, chi blasts are so easy to do that people can pick them up with practically no effort, even total losers (like Sean and Dan) or people with no training beyond watching videos (Sakura). It strains credulity to the utmost to say they're working on the same assumptions; how can Herb be more powerful in chi than any human (specifically stated) when his best stuff is no more impressive than Ryu's super attacks?
Of course, this can explained away. Theoretically, any incompatibilities can be. But it's hard to do it smoothly and believably, and becomes exponentially more hard as you add more series'. Eventually the weight of the conspiracies and "Oh, well, these old masters just didn't know anything about these techniques that dozens of people in this series regard as commonplace" and the other kludging adds up.
That being said, few people other than Link think the world is artificial or somehow unreal. Most people probably assume there was a conspiracy, or forces moving that nobody knows about just yet that kept the delicate balance of the world until they abruptly broke down.
As for the time travel device (you mean the Black Moon?), it's an interesting question indeed, which I am unwilling to answer due to spoilers at this time. ;p