It doesn't sound like Seth is Vertigo. Besides, he's been done already, and he's not a new character on the show. Dr. Alchemy perhaps? Edit: I was looking at Wikipedia and saw his name listed as one of the Beetle's enemies. But upon further reading, I realized he's one of Flash's villains.
Yes, but it has been revealed that a young Bruce Wayne (12 year old I believe) will be part of the story.
But why? And, unlike with Sarah, people would probably notice such a prominently featured character being played by a different actor. That's leaving aside that he's dead, though. I suppose that's one reason to bring in a new actor to play him, but I still think it'll come off as a copout.