I'd say you're spot on except for point (3). There's a number of crdt and event ...

I'd say you're spot on except for point (3). There's a number of crdt and event log approaches that, when combined properly in order to preserve user intent, can solve almost all merge issues of applications that do not require strong consistency.

> 4. What happens when the user has millions of items?

Partial replication is a problem I haven't seen many people solving but it is definitely the next frontier in this space.