A lot of those issues of ‘multiple primaries’ can be resolved with intelligent data types and actions. That is, if we have a notion of how the data is organized, a lot of decisions can be made a priori. Ones that can’t can be read-only during a split.

Comment groups are mergeable sets. Any unique comment is a valid comment.

For any individual comment, any tombstone causes a comment to be unseeable (and ideally be deleted). Any edits are latest-wins.

A lot can be sorted out that way - enough to be usable. Some databases even support that on a db level.