Question #1 The 12" frost proof hosebibb should work just fine for your application. Question #2 I would install the boiler drain valve after the pressure regulator. Question #3 The fluxes and solders that are used today are lead free and won't hurt if you ingest some residue from your pipes. However, I always flush my lines for a few minutes to clear what I can from them. Some plumbers are still using non approved solders and fluxes. I still carry some with me that I use on copper heatlines and drainlines.