That's all great to hear.
The coolant was the biggest concern, so if there is an explanation for that, the others may be solvable one at a time. The serious under-filling could cause the suction due to a big hot air pocket that was in there when you put the cap on (the air needed to be hot). When that air cooled, it would shrink - not as dramatic suction as condensing water vapor (where the volume basically goes to zero), but could produce what you saw.
On the belt -
are you 100% sure it is routed properly?
Fan pulley nuts (the 4 small ones) on tight?
You previously mentioned the Power Steering - when I did mine, the PS system stayed closed the whole time, and it and the AC were basically untouched while I worked around them. But I did not do anything special to protect them. Hosed everything down, etc. If you did not open any hoses, I doubt you could have caused a problem.
I did have a couple of minor belt squeaks after my HG work - partially fixed by a new drive belt tensioner and then fully fixed when I put on a special belt designed to handle slight problem situations. I guess when you touch enough old stuff, it's not unexpected to throw a couple of things off.