1) They can't lock your phone. Once unlocked, always unlocked.
2) PAYG gets the same towers as prepay. You don't get as much data - but when you're getting it you're getting what you get. (I can't get LTE with wifi on with a weak signal - with prepay. That's the phone.)
3) No tethering. Period. They'll cancel your account if they catch you.
OTOH, if you ever need service, they're about the best in the industry, and Asurion (that's the insurance they use) is now "it doesn't work, we replace it". Water damage, stepped on, caught in a slamming car door, manufacturing defect after the warranty runs out - pay the copay. I think it's available on PAYG.
But find someone you know who's on AT&T and check coverage in all places you're going to need coverage. There are surprises. (Sprint actually has better coverage here - except in one small spot, within the walls of my daughter's house. On the front step, fine. Back yard, fine. Side yards, fine. Deck fine. Inside the house, no signal. AT&T and Verizon? Fine signal inside the house.) Make sure you want to go with AT&T before you do.