You can't. You need to start a new game :(
you have to beat all the trainers at least two[2] times
yes
you cant
no
Yes
No.
dungeon leader can be any Pokemon you recruit. you will be able to change leading Pokemon after you graduate .
you can't (I think)
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.)
In the Pokemon Mystery Dungeon games you are a Pokemon. So, yes you can be a girl, but it does not change your appearance. Your gender and your nature (which is found out by a test) determines what Pokemon you are.
You can't.
Yes but after you complete the game. Including darkrai.