Steven will be in Silph Co. after you beat Trainer Red(Get all 16 Badges,Elite4) on Mount Silver. He will give you 1 or the Hoenn Starters at lvl 5.
Only in an event.
The Embedded Tower.
See related link.
In Pokemon SoulSilver, Marshtomp evolves into Swampert at level 36. See the link below.
You can trade one from another game, in soulsilver you can find a trainer with a chimchar, then go to the GTS and evolve it.
By using the TM Flash
You can find them in tall grass and catch them, so yes, you do get to see them.
No, neither did i :|
it allows you to be able to see the leagendary lugia
See related link for details.
No You dont
beat Clair then go see professor elm