The point about matched belts is a good point, sets I buy are often different even from the same supplier/maker, only a slight difference but enough.
I suspect that something is partially seized/seizing, almost definitely the alternator bearing, does this feel nice and free with no roughness, we know the crank isn't seized or the water pump(but check this also), if too much force is needing to be transmitted through the belts they can do what is happening.
Next time it squeels, stop and take belts off and check alt and water pump.
tabetha