Quote:That's what the liver is for.
Because injecting random chemicals into your bloodstream is so healthful
The alien toxins would only be allowed free range in the bloodstreams of the adapted humans if the precursors were grossly incompetent, since that design would require re-engineering every cell in the human body.
It is considerably more elegant, and efficent, to soup up the liver so that it can detoxify the alien biochemicals, turning them into something useful. Cooking, and careful choices of food, will deal with the remaining problems.
Considering what the liver already does, all the exotic chemicals it can already handle, this requires only minor alterations to its workings.
Engineering humans to use less iron, or other potentially rare elements, is excessively ambitious, meaning a massive redesign of our biochemistry. The elegant solution is to create a plant which concentrates the needed metals in its fruit, along perhaps with various vitamins.
This approach is easier, and has ample plot potential. Consider the possible cultural and consequences of a single plant which is vital for good health. It would be the centre of religions, and the cause of wars.
The most elegant way to do the genetic engineering is probably to put the extra genes needed in an artificial intracellular organelle with its own genes - like the mitochondria. Doing this doesn't create species barriers, but - since all your organelles come from your mother - it would mean that crossbreeds would always be the mother's race, never the fathers - which could have plot relevant consequences.
Note: if the chlorine worlds are the result of tampering by an elder race, then there could be other remnants. Imagine a ship, the size of a moon, which can produce elemental transmutation rays, fusing all the oxygen on a planet into chlorine from a distance of a billion miles. Imagine what happens if someone finds such a ship, and tries to reverse engineer the technology.