Zone 2 and A/B switch are not the same thing. If you power zone 2 from the receiver, you are using a separate set of channels completely, while if you use A/B you are using the same channel and you COULD cause damage, but it will be more likely to be to the speakers not the receiver. If the impedance load is too much for the receiver, it will likely shut itself down before any damage occurs to it, so you probably don't have to worry much about the receiver. I would definitely use the zone 2 option rather than A/B.
If they are going to be in the BR or very near the shower, then yes, I'd probably go with outdoor speakers.