I'm going to say it again:
Mxfrank, your posts make my life educational and entertaining.
A genuine two-for-one.

The correct belt is 10mm,the 9.5mm belts were dropped many years ago.
Belt tightness,one inch deflection under moderate pressure.
Alternators can make noise under heavy load,humming or type of squealing.
Warning on most alternators,Battery must be fully charged before running.
Why?Alternator may over heat,was not designed to run at max.for a long period of time.
A voltage/amp test of alternator is needed possible also of battery.
Remember new does not mean good.Testing is required.

Unless the vee portion of your current belt is sitting on the bottom of the vee on a pulley. You have over tightened the belt and still have a noise. Sounds like it is other than a belt issue.

If you take a bar of face soap and rub it on the vee portion of the belt with the engine off. If there is still a belt problem with the belt having decent tension. You problem is elsewhere usually.

It still sounds like the alternator is working far too hard. Yet the output is not showing up at the battery. Exciter voltage somehow is too high? A disconnect of the wire might make a difference. You have not boosted with a jump start in reverse polarity and shorted the diodes inside the alternator?

Did the alternator come with a new regulator?

Did the tensioner pulley bearings check good?