I have a 2002 Mazda protege that sqeals horribly belts have been replaced as has the alternator but the squealing persists now I've had someone watch as I turn the shearing wheel either direction and the belt completely stops turning I believe it is the power steering pully but idk if my diagnosis is correct.

Try spraying some WD40 with straw behind power steering pulley between pump and pulley if noise goes away the bearing in the pump are going bad. You can also do this with the other pulleys to see if the noise goes away.

I have put Preston's belt dressing on it and it stops the noise somewhat then after a few minutes it starts squealing again then it turns into a grinding metal sound if I turn the air on it stops

Ok that sounds like the A/C compressor clutch not disengaging all the way when turned off or the bearings in the compressor are bad can not actually say unless I could see and hear for myself. You can get the clutch changed with out buying another compressor but it has to be the right person that knows what they are doing so that it will not continue to make that noise but if it is the bearings you will have to replace the A/C compressor.

Findingsomeone who knows what their doing is hard