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.
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.