Some of this info I am sure you have done but its possible you may have missed some.
1. lots of WD40 on working parts you can get to.
2. the black "spring" that holds the nylon wheel in place needs to be held open some how to be able to freely turn the nylon wheel. Possibly remove that wheel. you should have a metal cam with a holder similar to the nylon one that is not allowing you to release the belt. you need to get to that and hold the catch open to turn it.
sounds easy and it is, but be careful if you find a spring it could decide to escape its happy home and ruin the belt mechanism. It is a lot of slow work but it can come apart and release the belt wheel. just go slow or avoid all the work and buy new ones. LOL
55birdman
55Birdman
Hickory NC