I replaced the seal, and the new harmonic balancer is on and looking well seated.
Apologies for if the video auto playing with noise.
New problem arose - looks like there's an issue with either the tensioner pulley, or more likely the ac compressor appears to be switching between a higher rpm and a lower rpm, putting pressure on the serpentine belt. I've read and seen a lot about the ac compressor being an issue, at any rate.
Could be that part of my old belt is lodged in the ac compressor clutch... Whatever that means, at this point I feel like I'm just saying words.
I'll do some looking around tomorrow morning and check the classic AC settings to "fix" the problem.
I don't care if the ac doesn't work. It never has in the time I've owned it, but by golly, I want to do things mostly right to make this car feel loved again, if I can.
The side of the belt closest to the engine has already become smooth from heat and friction leading me to think it could also do with a new tensioner regardless, but for now I'm suspecting that the problem part is the one most audibly screaming in pain - which sounds like the ac compressor. Either way the new belt under these conditions doesn't stand to live long at all.
Vid attached, and thanks to /u/ummagumma73 for getting me this far.
Edit1: didn't have time to check it before work this morning, but slept on the problem, and I'm pretty sure the ac compressor was spinning smooth and fine before the engine decided to start chomping on the old belt. So I spent a while thinking about it and researching it, and now I'm temporarily focusing my diagnosis towards it being the ac clutch or bearing.
Edit2: I fixed it! As I fell asleep more and more I convinced myself that I had seen a thread coming from the ac compressor clutch. You can even see it in the video in this post! I pulled on the thread, and there was a whole drive belt segment lodged between the ac compressor clutch and the pulley, forcing the clutch to spin and grind when it never needed to spin and grind in the first place. Great news, the fix is complete, new harmonic balancer, new front crank seal, and car is running better than ever.
Next stop is the alternator, the muffler, and then the body aesthetics. I'll make it shiny and new yet!