Have you tried remove the alt and spin by hand to check some friction, contact or weird feeling when spinning? May have not necessary remove the alt but just loose the belts and feel it.

I had once my alt giving up to 18volts at iddle ( more giving RPMs ) but no charge on ammeter ( some charge when giving RPMs ). One of the stator lead to diodes was making contact with just one of cooling vanes on rotor when spinning. Hard to feel but I was able to feel a small "stop" similar to the elect distributor stop due the magnet with reluctor points when spinning. Dissasembled, bent the wire a bit and done.