After you catch the Pokémon you have migrated, they'll be the exact same and once 24 hours is up (for Diamond, Pearl and Platinum only) then you'll be able to migrate again.
You would have to start a new game and catch the Pokemon again.
In Pokemon Diamond, Pearl, and Platinum, you cannot 'recatch' Dialga, but you can have multiple attempts if you save before battling Dialga. In Pokemon HeartGold and SoulSilver, when you do the Arceus event, you automatically obtain your choice of Dialga, Palkia, or Giratina.
you cant because you always have to have one in your party and migrating all of the Pokemon in your box would be really dumb.
if lugia faints then battle the elite four and he'll reappear
you cant get it without migrating.
you can get mew and mewtwo by migrating it from Pokemon fire/green
No.
no
yes.
by migrating pokemon from your gba game
you cant
you have to migrate it!