OneManga shutting down

[Epsilon]

Nope, trademark is quite different and it works differently. some things are both trademarked and copyrighted (Mickey Mouse), just trademarked (3, the phone company forinstance) or just copyrighted (Lord of the Rings).

Copyright gives you the right to control derivative works, which is why for instance fanfiction.net is possibly illegal (definitly operating deep in the grey zone of the law).

Not quite true.
The copyright law does, in fact, give the copyright holder the right to control derivative works. However it defines derivative work as:

"copyright"...includes the sole right

(a) to produce, reproduce, perform or publish any translation of the work,

(b) in the case of a dramatic work, to convert it into a novel or other non-dramatic work,

(c) in the case of a novel or other non-dramatic work, or of
an artistic work, to convert it into a dramatic work, by way of
performance in public or otherwise,

(d) in the case of a literary, dramatic or musical work, to
make any sound recording, cinematograph film or other contrivance by
means of which the work may be mechanically reproduced or performed,

(e) in the case of any literary, dramatic, musical or artistic
work, to reproduce, adapt and publicly present the work as a
cinematographic work

So basically translating, adapting to a movie, play, recording etc is all includes the rigths of derivative works.
Noteably the derivative works does NOT include pastiches, parodies, sequels and so on. You are right to say that these areas are grey areas. They are not covered by copyright law technically. Only if the work is sufficiently un-original is it covered by derivative works. However the copyright holder may be able to sue you under these laws or may not. So writing a fanfic may be a derivative work if it contains insufficient creativity, and it may not be.
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Epsilon