1. I would insulate the piping and the wall cavity. I would also get no-freeze sillcocks for both the existing and new bibbs.
2. Ball valves are designed to be soldered as-is. Nibco does offer these caveats: Solder with ball open, to protect seats. Wrap a damp rag around the body of the valve, away from the sweat socket. Today's solders require a temp of around 460º and the teflon seats can be affected starting at about 500º, so be careful. MAPP is a good choice, because you want to heat quickly, solder, and be done. If you have to continue applying heat for an extended period trying to achieve melting point, you cannot prevent the heat from spreading.
In general, I suspect you will be less likely to damage the valve by soldering in place, than if you try to disassemble and put back together.