Hmm - I think I've answered my own questions to some extent:
- Yes, it looks decent enough. Probably not a lot better than the rest of them but good.
- No Cmk file, but making one isn't too hard. The layout file format is easy enough to use.
- That doesn't really do a world of good though: The lessons are tailor-made for layouts, and obviously Cmk has none.

Turns out you can even use it without making a file: If your comp has a Colemak layout installed you can set a virtual-key layout which uses your mappings! Nifty that, but as mentioned it won't give lessons based on it so meh.