first things first, take some brake CLEANER and clean up the whole belt, all the pulleys etc, anything the belt comes in contact with. I had a bad belt squeel for months and tried everything just like you did and it was only getting worse. then when i had a recall done on my truck the mech said all that grease and stuff was just making it worse and he cleaned it with brake cleaner and like 95% of the sound went away. it still squeeks a little but i think thats more because theres probably a pulley slightly out of alignment because ive taken apart the front of the motor and didnt really have the time to check for alignment.

 

...but wouldn't brake cleaner dry out the rubber, therefore wearing down the belt?

 

it wont even be on there long enough to do that because it will dry up... if you soaked the belt in it, then it might. if it makes you feel any better wash it off with water.