1. No. I didn't use Google + at all in the beginning, and the gallery works fine. I think what is being confused is that with Google + enabled you can have the phone automatically back up all your photos to G+ (privately, not visible to others). But the Gallery is a stand-alone app that doesn't need G+ at all.
2. Are you saying you plan to buy the unlocked version outright (not from AT&T) and then put an AT&T SIM in it? If so, then yes, you'll be able to get updates directly from LG. I'm not positive if these updates come over the air (OTA) or whether you'll need to install them manually. I think they'll still be OTAs, but am not positive.