It turns out that if you set a checkpoint while you're on a conveyor belt, you'll never be able to reset back past it, because once you reset to it you immediately move away from it.  Perhaps the game shouldn't allow you to do so?  In the meantime, I'm pondering either starting over or giving up, alas.

Thanks for letting me know! I looked into that at some point but I also rewrote the checkpoint system a few times so I’ll check.