1. XD Possible on Emulator. FRLG are possible, but through trial and error AFAIK (no idea about on cartridge I'm afraid).
2. Not particularly I'm afraid.
3. Incredibly easy. Check out
this guide for breeding in Gen 6 (short story: new item forces 5 IVs to be inherited instead of 3 making it much easier); and
this guide for resetting legendaries (short story: they get 3 randomly selected IVs at 31, making it significantly easier to get good ones).
4. Nintendo shut down their servers for Wi-Fi for games before Gen 6, meaning Pokécheck no longer functioned as desired; hence the admin took the decision to close it. There is no information on when (/if) it'll be available again, please
do not ask (it's a rule in the OP of this thread).