You are entirely right that the data structure is paramount. But I'm not using Xanalogical documents nor any front or backend designs from Ted. I actually think he didn't do a very good job with those. Instead, Mimix records a user's interactions in MSL, a domain-specific language (not Turing complete on purpose) which describes a forward-write-only experience of reading and text composition.

I'm glad you brought up the UI because it's an area ripe for innovation and one we're seriously excited about. UI is one of my specialties and I'm looking forward to sharing our designs for interacting with canonical documents in a modern and approachable way. HN is my favorite place to post this kind of stuff, so you'll "Hear it here first."