you cant youre talking about mystery dungeon
No.
It is not possible to get Mismagius by yourself in Pokemon Diamond. The only way to get a Mismagius in Pokemon Diamond is to trade one from Pokemon Pearl.
In pokemon diamond and pearl
You Transfer It From Pokemon Emerald To Pal Park ANd You Have Yourself A Lombre
there is actually no finish to Pokemon diamond as you can keep trading and catching Pokemon. however, the actual storyline finishes when you become Pokemon champion.
you cant
No
capture the legendary and train your Pokemon
You actually can't really become Mario in the game without editing sprites, which are the pictures you see in the game such as the Pokemon and the players, including yourself.
bye dying
find them yourself.
in pokemon pearl,diamond or platinum you have to eight badges in pokemon black and then trade kadabra to you from yourself and it will evolve