OP here with another update.
All was going well for a while with my left side high hamstring pain. I'd built up to about 35mpw, had a long run of 10-12 miles in there every few weeks, felt strong, happy, grateful, and confident in my management system.
Then winter hit and brought the usual ice/slush/snow. I headed inside to the treadmill a total of four times over the course of two weeks (continuing with runs outside on days with good conditions). I kept things pretty conservative on the treadmill but might have gotten a little carried away with the pace on a few of the runs. I kept the incline at 0% for these four treadmill runs, which I think may have been my most critical mistake. In the last two treadmill runs I noticed some tightness behind my left knee (but no pain up in the left high hamstring/butt area where I'd had pain before). The next day on a run outside that included some downhills, my left leg gradually tightened up behind the knee and in the lower belly of the hamstring so much that I had to shuffle home and cut the run short. I figured, no problem, take a week to rest and resolve whatever this "new" issue is.
Annnnnd I haven't been able to run since December 28th. It's officially been about three months. This "new" injury won't go away no matter what I try. My left side "high hamstring" issue has seemingly completely evaporated -- I can't get that pain to activate any more even with the most aggressive motions that used to light it up. Now it's been replaced by this left side belly-of-the-hamstring/tightness behind the knee nonsense -- I can jog for a few minutes, but then it gradually tightens up so much that I have to stop. If I hit any slight downhill or decline it tightens up way faster. Running uphill is completely fine -- if the hill is steep enough, I don't feel the hamstring issue at all (which is wild, since this used to be a major problem). Elliptical, stairmaster, hill running all fine -- seemingly since they keep me out of the portion of the range of motion that ignites the tightness.
I've tried several weeks of rest, glute & hip strengthening, chiropractic, more PT, heat, nerve flossing, super focused meditation/visualization of my stupid injury healing (can't hurt, right?), deep tissue massage, and a standing desk. Nothing has helped.
For a random few weeks during this time period, I also had the pleasant sensation of water dripping down my left calf while I walked sometimes. I kept thinking my water bottle in my backpack was leaking on my calf, but then I'd reach down to wipe it away and realize there was no water. I also had a lot of tightness in my foot below the fifth metatarsal, though I'm not sure if that was related. We treated this as a potential mild bulging disc issue and with a few weeks of cobras/sphinx poses the trickling water sensation down the left calf and tightness in the left foot don't seem to be a problem any more. However, the weird hamstring tightening sensation remains.
Seems consistent with some kind of sciatic nerve/piriformis/etc issue but I have no idea how to fix it. I now find myself pining for the days of high hamstring pain since I had figured out how to manage that iteration. The thought of seeking out more professionals to help me is so discouraging. I don't think I can take any more useless ART and glute bridges and monster walks for this.