Maybe this will work. Get a blank AT&T SIM. One should have come with the phone if you unlocked it yourself.
Wipe the phone to stock state, put the AT&T SIM in, go to the Apple store, tell them you used the phone for a few weeks, decided you didn't like the coverage from AT&T,
AND your screen was messed up, so you cancelled your contract, ported your number to T-Mo, got a new phone and planned on selling the iPhone.
Then tell them you got worse service with T-Mo, and started to miss the iPhone's features. Then say you want to port your number back to AT&T, but the phone needs replacing because of the lousy screen, and you want to wait till it comes back from repair/replacement before you cancel your T-Mo service.
This way you can explain the used state of your phone and why you don't have an AT&T number all at once.
Practice your story in front of a mirror a few times.
It's a little sneaky, yes, but no worse than them not honoring a warranty because you did with your hardware as you please.