Well, I managed to kill my phone for good working on getting 4G working. (I'm going to remove my post, so nobody else runs into any problems).
The good news is: I got closer to getting the UICC recognized. It was registering with MetroPCS (and LTE, I believe), but would freak out and shut down RILD. It keeps doing that. However, it was definitely being read, and was definitely registering with MPCS.
The bad news is: I tried to revert back to stock, and I have no service. Oddly enough, my 4G data is working, but my MDN, MIN, and PRL are all screwed. MDN, MIN settings won't stick, and the PRL reads 40000. I tried using CDMA programming tools (QPST, CDMA Workshop), but nothing sticks. The behavior is similar to the LG Esteem getting stuck in "test mode". Unfortunately for me, none of the tools (LGNPST, Flash tool, test mode repair, etc) work for this particular phone. Maybe it's too new, but the dll I found didn't work (MS870.dll).
(And I tried unbricking. I did it several times, but it didn't change anything. Didn't even really "unbrick" correctly.)
I'm not very familiar with CDMA programming, so if anyone has suggestions, I'm all ears. Until then, I'll have to find something else (Premia or Motion). Keep up the good fight, guys!