[stcredzero]

> IMO, from having studied this for many, many hours over many moons, is that what Apple is trying to do is fundamentally hard, if not entirely unfeasible.

"Distributed Databases are Hard" has been a mantra for many decades, probably getting close to a half century. We have known, basically forever, that this is hard on a fundamental level. (Google "Two Generals" problem.)

> Apple was hoping that by shoving a bunch of diffs of your database onto the server, that clients can reliably reconstruct a sane database by playing them back

This is called a variety of things. One of the terms is "transactional replication." It can get very very sticky when there is more than one party originating write transactions, though.