What I think is missing is not the description of a specific protocol,
but a higher-level spec of basic capabilities needed for the
collaborative editing support in Emacs. Is this available anywhere?
If not, could someone please write it up?

For example, one thing that strikes me is why "collaboration" via a
dVCS is not a good solution, or at least the basis of a solution? Am
I missing something?

Collaborative editors usually show modifications done by other users in real time. I don't know how major conflicts are resolved.

How would you emulate this with a VCS? Commit-push-pull with various --force flags, on a timer run every second?

Thibaut