Hard of hearing here, having a little trouble keeping up.
Couldn't quite hear what you changed but I'm guessing you are now happy with 5Ghz (and, with your new AC router, whatever you choose, the consensus seems to be, that far corner should get slightly better?)
While 2.4Ghz improved it still seems a bit uh disappointing downstairs? I think the focus should be on why 2.4 performance is so poor through the extender (which is direct connect to the router). Being direct connected that extender is almost like having a router downstairs. You really should have gotten near service (90/12) speeds through that.
Whatever you turned off upstairs seems to have helped both bands? (Better performance through closed doors seems counter intuitive : -) What else, what else is downstairs that could be affecting 2.4? Xfinity? Oh, you've the spare extender in the garage. Guess you could swap extenders on the off-chance there's something wrong with extender?
Edit: Replayed your video. You did shutdown Xfinity? (Sorry, having trouble hearing.)
Yeah I turned off xfinity. That seemed to be the culprit actually.
Yes, I can't explain why that powerline ethernet extender is so darn poor on wifi. when you plug into it, it's excellent. My only other idea is if I put my old netgear WNDR3700 router downstairs and use a powerline ethernet adapter to feed it internet. With that setup, I think I will have acceptable 'n' performance...
but I'm not really sure how to effectively bridge it. Do I name the SSID same? do I call it HHC_D (for downstairs?) and make sure all downstair devices connect directly to it? hmmm