Spray the webbing of the belt with silicone spray. It will free them up, but the frayed bit I can't help you with. But you can change just the webbing, remove the mechanism from the door pillar and fully wind out the belt, you will see the way the belt is retAined. Remove and replace, you probably can use a commodore webbing or something common. As after the web is removed, just slide buckle off the end.
Easier to wash or dye when they are removed too.
I know there is a compliance tag at the fixed end of the belt, but I can't remember what it says, replacement belt will need similar tag