Duplicati tries different ports, starting with 8200. The first user to log in will get that port, the next will get port 8300, next on 8400. The port number is recorded in the settings database, such that the next launch will attempt to keep this number.
This can go wrong, say that Kees is 8200, Ken is 8300, but then the machine reboots and Jon logs in for the first time. He will then get 8200, and when Kees logs in, Duplicati will see that 8200 is taken and scan for a new one, finding 8300, etc.
If the users have bookmarks, then it all breaks.
Yes, all users in the normal setup have their own server and thus their own scheduler.
Honestly, I do not know how many are using shared-system machines and performing individual backups. But given that it is slightly cumbersome to set up, I would guess: not many.
Yes, that is also the plan. It requires me to modify the MSI packages, and they are time consuming to work with. But eventually there will be a radio button that says “install as service” / “install individually”. This will also make it much easier for users to enable VSS snapshots.