ok I read another question on this site just like this but i will answer it again. deoxys changes formes every time you go up or down stairs.
You don't. IT IS ONLY POSSIBLE IN EXPLORERS OF SKY!
yes
you cant
no
dungeon leader can be any Pokemon you recruit. you will be able to change leading Pokemon after you graduate .
Yes
After you beat the game, you will have the power to change the leader at Chimecho's Assembly. You can not change the leader in a dungeon, though.
you can't (I think)
No.
you have to beat rayquaza to start evolving and having different leader pokemons.
You can't.
you can't sorry man, but it ain't gona happen!
Yes but after you complete the game. Including darkrai.