how to force a re-scan of media

[Nadahar]

You can't give UMS "commands" from the command line - there's no mechanism to receive such information. You can use the "web console" (if you can find it, look in the log for the address) to trigger a rescan. It's not possible to scan new media only specifically, although this is in theory what is always done. The file path and the "last change" timestamp is stored in the database. The shared folders are scanned for files and folders, but the media itself isn't analyzed if an entry with the same file path and timestamp already exists in the database. Instead, the information in the database acquired in the last scan is used.

It the timestamps don't stay the same, or the paths vary, a new analysis will be triggered though.

[Nadahar]

You can't prevent UMS from analysing the files, or UMS would have no idea what to do with them. It normally won't read the whole file though, just enough to get the information it needs (usually just the headers). There are some files though (MP4 for example) that has the brilliant idea that they place the headers at the end of the file, which could pose problems. UMS won't actually "read" through the whole file, it will try to seek/skip to the interesting part, but this depends on that the underlying I/O actually supports seeking/skipping. I know that poor implementations actually just "reads and throws away" all the data behind the scenes when seeking, so if your rclone mount system is of this "primitive" type, it could mean large downloads.

Without knowing the details of the media files, UMS can't handle them, it's simply impossible, so UMS needs to get this information one way or another.