I finally got my phone! I called again to confirm that they were able to activate T-Mobile trade ins before I left and a very bored-sounding person on the phone said they had no issues, so I went to Best Buy.
I get there and I tell them I'm there to pick up a phone. They ask if it's thru T-Mobile, I say yeah.
They say that their systems have been unsuccessful all day in processing T-Mobile trade-ins, and that I probably wasn't able to get my phone today. I told them the guy on the phone said the opposite, and they shrugged it off. They said they could try anyway.
We went through the process, and they were telling me to expect an error to pop-up at the very end. They even had a guy from Apple there (wearing apple shirt + had an apple employee badge) who was helping them troubleshoot/observe what was going on. They verified the condition of my phone, signed out of Find My, and did all the activation steps at the register. At the very last step when they were expecting the error, the activation magically went through and they were honestly in as much disbelief as I was, lol. I then reset my phone, handed it over to them, and left with my new iPhone! Crazy day but it worked out.
I don't know if the system got fixed right before I got there, or if I got lucky (they said after that they're going to start telling people it's a case-by-case basis). But the staff was really friendly and seemed knowledgeable/knew what they were doing so maybe that helped.