1. Not really an issue, as Perfect Timing is a Divination power and Tyranids can only gain access to powers from the Biomancy, Telepathy and Telekinesis disciplines.
2. This is yet another area GW have failed to consider. I would say roll a D2 (1-3 or 4-6) for it.
3. Correct, unless their attacks have the Armourbane special rule MCs no longer get an additional D6 for armour penetration.
4. Yes.
5. I don't quite understand this question. If what you're asking is, can Tyranids with psychic powers from the Codex exchange them for powers from the main rulebook in a friendly game or tournament, the answer to both is yes.