Well, it's been a long road but I can conclusively say that it is NOT the phone. Or at least nothing in the phone "broke." Sprint HAS to have made some change(s) somewhere on their end that makes this phone no longer work on their network as it was designed.
LONG story short, LG was fantastic with the warranty and I finally got a replacement phone yesterday (previously they just sent back the same phone I sent them). I activated the replacement on Sprint again, and guess what?! Same issue. Big surprise (not).
There are also now a couple more recent reviews on the phone on Amazon saying they had exactly the same issues. One review said he fixed it by changing the APN to cinet, like we talked about before in this thread, but the reviewer made no mention of how he was able to get it to stick with the cinet APN. As I mentioned in earlier posts, any time I try to switch to cinet, it flips back to n.ispsn after a few seconds. Last night I tried adding a separate, custom cinet APN config with different values I found on the internet, but it still never worked.
Sprint is useless at this point. Not their phone so they won't touch it. I wish they would even at least acknowledge that network changes were made and "here's why your phone no longer works," but they still insist that no changes have been made.
I'm likely leaving Sprint anyway. I've thought about leaving for a long time, they have terrible coverage. The only reason I stayed is that I had a great, old, cheap, grandfathered unlimited data plan that nobody can touch price-wise if I give it up. But I think this is the last straw.