Replaced radiator & hoses, all hell broke loose! (2000 EJ8 — P0505 and noise)

Imgur cut my video off at one minute but I’ll link it anyway since it shows off the noise and behavior I’m talking about: https://imgur.com/a/DH8FXj3

I finally finished replacing the radiator, thermostat, overflow tank and heater hoses on my 2000 civic EX (non-ABS automatic, 2-door coupe, 260k miles). No leaks!

First, some stuff from before this repair: The car has had a hunting idle issue for the past year at least, but it was never bad enough to make me do anything about it and it never set a code. A month or two ago I noticed a burning plastic or burning electronics smell that was detectable both under the hood and in the cabin, but it went away after a few days. I noticed some noise from the accessories at this time too but it wasn’t so bad

I bled the coolant system while it was still jacked up for a bit. While bleeding the coolant system I noticed a rather distinct metal-on-metal squeak coming from the engine near the driver side (in the video). This noise is different than the one I heard before.

After getting it off jack stands after a few weeks of sitting in my garage, I take it on a short set of errands around town and see a check engine light. I take it to shell to use their self-serve scanner thingy and I get back a P0505 (Idle air control system malfunction). Driving home I notice some surging and bucking when applying slight throttle.

I read up on P0505 and I find out that, of course, it can be caused by air in the cooling system. While shopping for a new cap and hoses for the reservoir tank I notice in the parts drawing that there’s some kind of seal on the cap. I go down and check and sure enough it’s missing. I figure I might as well bleed the cooling system some more anyway since it doesn’t cost anything but my time.

With my car now parked facing uphill on my somewhat steep driveway, I start bleeding the cooling system again and only now notice the bucking at partial throttle that you can see/hear in the video I linked. I also notice that if you rev the engine it dips WAY down before coming back up to a sensible idle speed.

I bleed the cooling system for a good half hour but even after that point there are still a few very small bubbles coming up. The overflow cap seal is still missing so I figure I’ll wait until I have that in place and try bleeding again.

Now, I’m wondering if I broke something by removing the airbox and moving the intake hose around while working on the cooling system. I did have it bungee corded to the hood for a while. I re-checked and I didn’t see any electrical connections or hoses loose.

The other thing I’m wondering: The car’s been in my closed garage for a good 2 or 3 weeks and the temperature in the area has been dropping sharply for the past month. Could these problems have showed up because the car got a lot colder than it has been for the past few months?

Edit: more details
May or may not be relevant:
* I also changed the transmission fluid at the same time, dunno how that could affect it but figure I should include it anyway.
* I have noticed somewhat worse gas mileage recently but had just chalked it up to using the AC a lot