I think this warning means that schema is not properly compiled for some reason. That's why I recommended to recompile it earlier. I also believe the method will work in 22.04, I just don't have a suitable virtual machine with 22.04 for experiments.
By the way, marco is using this magic number -- 36 -- in marco/src/core/keybindings.c file too, and it is related to workspace count. I'd edit it there as well.
Everything works in 22.04. Thanks again for all of the help. And thanks for MATE. It's so elegantly unobtrusive that I forget it's there and can focus on the problem I'm trying to solve. That clarity is impossible on Windows and MacOS.