Need help triggering story events.

[SoulAscension]

Okay, so I caught all monsters in Pokemon Platinum and wanted to transfer them over to newer games, but the game is missing. I have Pokemon Diamond (basically a blank file now), and want all the exclusive Pokemon such as legendaries as soon as possible as I really don't want to have to play through the entire game again (I've played through it 3 times). I want all of them legitimately (albeit using illegitimate means) with Action Replay.

My question is: how do you trigger each legendary? I am using Wall-Walk to travel at the start of the game along with other codes to keep me safe. Would I be able to simply go to where Dialga is and the event involving Team Galactic will commence? And after capturing Dialga, would that automatically make the legendary trio appear? And what of the others such as Heatran and Cresselia? Do I truly require the National Dex for them to spawn?

EDIT: This thread garnered so much help that I was inspired to speed-play through the game, and have my legendaries. <Self-Close>

Edited July 29, 2014 by SoulAscension