Yes but after you complete the game. Including darkrai.
Yes
you can't sorry man, but it ain't gona happen!
Talk to one of the Shaymin in the village. At the top-right, I believe. It'll give you a Gracidea, which changes Shaymin's forme when used in a dungeon.
The mystery is every dungeon you go to, they change every time you enter them and so does the item layout, or the kecleon shops in side the dungeons.
Deoxys' Forme randomly changes as you progress through a dungeon. The changes are purely cosmetic, except for the Speed Forme, which doubles his movement speed until you move to the next floor.
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!
It depends on which Mystery Dungeon. the first one, yes you can evolve and go on, the second, you can't evolve at all, but if you have an action replay you can change your hero Pokemon into any Pokemon you want (you can do the same with the partner Pokemon, unfortunately, you can only change in dungeons.)