Squealing noise possible bad alternator, A/C compressor, or serpentine belt slipping?

Can't seem to catch a break on my vehicle listed above, recently I had to replace the timing belt and water pump, I replaced the serpentine belt with a Bando and not too afterwards started to notice a squeal that would go away after the engine warmed up, but yesterday with a heat index of 113 with the AC on this ear piercing squeal started and after driving around for about 20 minutes I started to lose power, I turned off the engine and couldn't start it again, luckily I had my jump box, no way would I be able to run the AC, suspecting the alternator since it looks like it's the original over 20 years old. My van more like an oven than van I went to AutoZone thank goodness their AC was nice and cold, to get it checked and at first the battery which is less than 2 months old was so low it wouldn't hold a load, so they put it on a charger, meanwhile thinking the belt might be slipping I tightened up the tensioner 30 mins after charging the battery, it started right up, and the squeal seemed to go away, the tester and multimeter showed the voltage at around 12.6 and dropping, prognosis bad alternator, cost for part replacement only around $250, thanks to inflation, went next door to Pueblo Auto Repair, got a quote for replacement, $550 2 hours labor.

I wanted to make sure it was the alternator and without taking it out and putting it on a bench there really is no way to know for sure, except the mechanic reminded me about the old trick of removing the ground wire while the engine is running, if the engine shuts off the alternator is bad I know there is a risk that goes with that but the mechanic said to make sure I made sure I kept it from connecting to any ground, I went back to AutoZone, removed the ground and the engine kept going it was surging though probably because the battery was low. I went back into the nice cool store, told them and they said it could be the voltage regulator in the alternator is bad. I decided to get a second opinion, but first I had to stop to get something cold to drink I went to the market, came back and it started right up so I went to O'Reilly's, less than a mile away, the analyzer showed the same thing, at that point I risked it and drove back to home base about 4 miles and went online to research the issue, I learned about 2 more tricks one being rapping the alternator with a hammer and see if the voltage goes up if it does it's a bad component, it didn't go up, two putting a screwdriver near the pulley, if its drawn to it there is a magnetic field and the alternator is working properly, now here comes the twist in the story I connected my multi tester with engine running and the voltage was 13+ and climbing I put the A/C, lights stereo on and the voltage held around 12.6 but when pressing the gas with the A/C running that horrible squeal came back, could it be the belt is slipping? I've tried all I can to tighten the tensioner it or could it be the compressor is going? Or could it be the alternator is going? Sorry for the long story but there was a lot to explain. Thanks for any help.