You could either use an Action Replay or a Nintendo event.
using a Pokemon event
Yes, you can trade them from diamond and pearl
You cant
I think you can only pass Pokemon from Emerald to Diamond/Pearl
The Membership Pass can only be obtained using an Action Replay. The event for this item has ended for Pokemon Diamond, Pearl, and Platinum.
it depends which man
Nintendo event
There is no such thing as a "rainbow pass" or there is some diffrent name for it.
You can get a membership pass by Nintendo event or mystery gift
Pokemon trainers don't know for sure.
see 450 Pokemon in the pokedex