"If the alt is seizing up causing the squeal would it do so occasionally or constantly?"

It is possible but not very likely. Easy to rule out. When it is hot, shut off the motor and quickly slip the belts off of the pulley. Spin it and feel the bearings. Give it a swift twist and it should spin on its inertia several rotations.

There are so many other things that will make that pulley slip, starting with the rubber bushings, and progressing through worn pivots, to engine oil vapor or leaks. An old glazed belt with glazed pulleys, and even a shorted cell in the battery will drive you bonkers trying to get it to play nice.
--
Art Benstein near Baltimore

Two fish swim into a concrete wall. The one turns to the other and says "Dam!".