The coolant level in the overflow bottle normally rises when the engine warms up and then stays at that higher level until the engine is shut down and cooled all the way down.
That's a lot of steam/smoke coming from the tailpipe in your last video. On a warm engine there should be zero smoke visible even for these old 2Fs.
What we sort of know is:
1. The engine keeps on consuming coolant
2. There's excessive steam/smoke coming out the tail pipe.
Normally 1+2=3, and 3 in a this case isn't a pretty number, but if the lucky gods are with you, maybe, just maybe the cooling system still is purging air (which would explain the coolant consumption) but that wouldn't explain the excessive steam/smoke out the exhaust.
If I were in your shoes, this is what would be going through my paranoid mind:
I wouldn't be sure if that was smoking oil billowing from the exhaust or steam (or both) but I'd assume the worse.
I'd assume that the head cracked from an overheat that wasn't noticed until too late. (Why was the cooling system way low on coolant if the car was in storage for 5 years?)
Cylinder heads are hollow inside and any crack, just about anywhere, if it's big enough, will allow pressurized coolant to get into the combustion chamber (and create steam).
Also as mentioned, a leaking head gasket at the right (wrong) spot can allow coolant to get into the combustion chamber.
Both of those scenarios suck, but the first one sucks way more than the second.
IF coolant is getting into the combustion chamber/s (creating steam) it's doing so because the cooling system is pressurized. It isn't getting sucked in, it's getting forced in by the radiator cap.
If the cooling system isn't pressurized, no coolant will get into to combustion chambers (unless there're a big hole in the head or head gasket). But exhaust gasses could still get into the coolant.
That is something that can be checked with a test kit.
This is what I'd do,
I'd get another radiator cap and dismantle its pressure release spring mechanism so that the coolant will flow freely to the overflow tank. Get rid of the spring in the cap. It's easy. Then screw it on the radiator neck.
Then make sure the radiator is full- then run the engine. - with an unpressurized cooling system (no it won't boil over).
Take a look at the exhaust after driving it for a while. If the steam/smoke is gone, then the head has to come off. Crack or head gasket issue.
If the steam/smoke is still there, the head still has to come off because an engine shouldn't do that.
