Unless you're referring to a cheat device code, there is no way to do this. You may be thinking of Pokemon Mystery Dungeon, in which you play as a Pokemon for the entirety to the game.
Yes, much like it's predecesores (Ex. Pokemon Gold and Silver could trade to Crystal) Pokemon Platinum will be able to trade Pokemon from Pearl and Diamond.
You could give it a soothe bell, feed it its favorite poffins, etc. . Just the usual, i guess
You have to battle other Pokemon with it.
you can get a plusle in diamond by migrating it from like emerald or something like that
Trade with diamond, Pokemon diamond only can get it from a iron ball like fossil
Yes, it's possible. Just do it like Diamond and Pearl.
there is no part like that in Pokemon diamond
shaymin is a special event Pokemon, fairly weak, and no you can't. you have to get like oak's letter or somethin and u get teleported to flower paradise. this is what I've heard.
yes it can be traded
As far as I know you can only get it with Pokemon that already have it. But i expect you could find one in dark places like caves.
493 just like in diamond