silver would be fine. I've never used Mapp, but from what I can tell you are much more likely to cook the joint with MAPP than with O/A. Brazing is all about heat control. If your torch can barely support the heat you need, that's not control, that's pointing the torch at the frame. A practiced builder will generally be moving the torch quite a bit, and you have to know when to point it off into space.

People say to strip the flux off of the pre-fluxed rods and then clean the rods. I don't know if that is necessary, I haven't really tried pre-fluxed rods that much. Unless you are sure you are using compatible fluxes, it's best to only use one.