I would encourage rolling your own crypto for fun, because, well, it is. But yeah, you should maybe also make clear that people should not use it, because that is rule 1 of cryptography.

I am afraid that means that we are significantly less than 2/3rds there

Does this not collapse at some point? Like you see with running an image through image to image multiple times?

People not adopting standards quickly and correctly is annoying. In no way is this the fault of webp.