Something seized...? :rolleyes:

Which was hmm... quite obvious from the first video don't you think.

Listening back to the video, the sound at the beginning is the fan clutch being all caught up with half a belt tangled in it!

In hindsight... sure! ;)

I've replaced the water pump belt at this point, and it resolved a portion of the sound, but not the idling and rpm issue.

Belts don't fail like that spontaneously - something caused the failure. Either a failing water-pump or alternator most likely, although an idler or tensioner is also possible.

It's also likely the new belt will suffer the same failure pretty quickly unless the cause is located and addressed.

Alright, one issue I noticed is the hydraulic belt tensioner seems to be loose, I can't tell what about it is really making it loose, but it moves by hand when bolted in. This is also a part I did not replace when I replaced the other pulleys and tensioners belts and water pump and thermostat

I'm done for the night, but when I turned the water pump, it didn't appear to be the issue, or maybe I don't know what its supposed to feel/sound like. I also didn't examine the other pulleys too well.

When starting the car is doesn't turn over as easy as it always has.

Next time you get in there, remove the new belt and check that the alternator pulley turns freely. Loose is not your issue - stiff and binding is.

If it's so tight as to make the belt squeal and it makes starting the car harder, it's likely a seized alternator bearing - since you have already eliminated the water-pump.

If it IS the alternator, I doubt you'll be able to turn it by hand (and you should be able to do so).

Just went back out and turned the alternator pulley like butter.

The water pump actually seems super suspect now, It does make a squealing or moaning noise when I turn it, alongside a metal or bearing noise. Is that normal...?

Could me driving with the broken belt have damaged other components..? To be fair the belt was still half intact on the pulleys, the other half had gotten eaten up by the fan clutch.

https://www.realoem.com/bmw/enUS/sho...diagId=11_2190

Looking at the referenced link, it appears like part number 4 is somehow loose, when I grab the pulley part number 16 it wiggles the whole unit.

I also observed when disassembled, there is a great deal of play in the part number 8.
Initially I was thinking maybe the washer needs to be replaced ..? When I reinstalled the unit I felt some improvement, but after running it I noticed now it is loose again.

Sounds like the water pump has seized. Bearing weep hole on water pump has leaked coolant. And eventually the belt has disintegrated due to heating up and too much strain and slippage.