[strus]

Arbitrarily choose an option, but expose the fact that you've auto-resolved a conflict and allow the user to manually re-resolve. This requires even more UI work than option 1.

This is what every "cloud file sharing" provider like Dropbox is doing. If there is a conflict, the version on the server is "the right one", and your locally conflicted file is copied on the side with some annotation in the file name.

[GregorStocks]

Yeah, but Dropbox is sort of playing on easy mode, because the data is "just files" and you can manually resolve the conflict with regular old text editors, etc. If you don't expose your app's data model in the file system (and on a phone you generally wouldn't), that means you need to write something custom to resolve the conflicts.