Some additional comments:
to 1: Okay, so I discovered that Sync does update the repository, but the changes don’t always appear immediately. For example, one of my colleagues submitted changes to the landscape in our main map (opened by default when the project is opened). Yet when I used sync, nothing changed in the viewport. I waited a bit, played the game. Nothing. Then I exited the editor and opened again and lo and behold, the landscape was changed.
to 3: I think that was just me trying to diff the first version in history against its previous one, which obviously didn’t make sense. Diffing against previous version appears to work fine.