to 3. Be sure to use the same folder id, or you won’t be able to sync.
This will work. But as Syncthing can’t tell without “sync” that the content is really the same, it will seam that it syncs, but if the metadata (mtime, permissions, …) is the same, syncthing will do nothing to the files and should quickly show up to date.
Depending on the amount of data (size, number of files / folders) the initial index exchange can take some time and data will be transfered.
If it isn’t the metadata will be changed on one of the devices which shouldn’t produce conflics and will be fast as well, without much data transfer.