Tomu, being a ARM M0 core, might take some time to compute ECC signatures, but it is probably fine in practice.
Our code is designed to be small and portable, so I think it could easily be run on the Tomu. Just need some work to change the USB drivers stuff.
Not that I'm aware of. Might be trickier than U2F (that exists already) because FIDO2 is a bit more computationally expensive, and probably larger code base. But certainly an interesting project to add to the pipeline.