It's Capricorn that goes up in flames, and they are trapped in the basement or something.
I think you could be right about Miles. He fits the build of the masked figure they showed lighting up Nora's house. When he was in Marty's office he point blank said he has been institutionalized his entire life and isn't quite sure how the real world works yet (hence his need for a shrink).
But why would he try to set up Christian? It's too coinsidental. The turpentine, torching two of Vincent's buildings, the blue prints on his computer of Vincent's buildings etc.