I messed with several things to try to coax it back to normalcy and got nowhere, so tonight I decided to just find the proper RUU and go from there. (For completeness' sake, specifically it was RUU_A16_UL_M60_DESIRE_SENSE70GP_TMUS_MetroPCS_US_1 .36.1550.14.exe, which seemed to be the same version firmware as what the phone had already been running.) The process completes without error... but not really, because it still doesn't work. It did *something," because the Device Unlock app now finishes, (seemingly) and if you have a Metro or TMo SIM in, you can even run a Search Networks scan that seems correct and says it connected successfully... but the signal level remains zilch.
In fact, it seems like in addition to the general connection problem, that the device unlock isn't *really* working for some reason. No error comes up when an off-network SIM is inserted, but I noticed that for one, the APNs do not generate the way I expect them to on a properly unlocked device - not even T-Mobile will autofill, just Metro. This *could* be normal on this device, but I don't think it is... and regarding the "Search Networks" scan, there's a reason I said "if you have a Metro or TMo SIM" earlier. The option is greyed out entirely when any other SIM is inserted. Hmm.
So what the hell is this, any ideas?