it works like this:
tmutil deletelocalsnapshots <snapshot_date>
bash-3.2# tmutil localsnapshot
Created local snapshot with date: 2017-12-14-172436
bash-3.2# tmutil listlocalsnapshots /
com.apple.TimeMachine.2017-12-14-170118
com.apple.TimeMachine.2017-12-14-172436
bash-3.2# tmutil deletelocalsnapshots 2017-12-14-172436
Deleted local snapshot ‘2017-12-14-172436’
Great! Shouldn’t be much of a problem to put together two scripts with this I guess.
Well, have an iMac with High Sierra now for testing, hoping to find some time next week.
Dunno how to mount a snapshot, yet - or, how vs. where urbackup needs it?
It’s probably mounted already. We just need to find the path where it is mounted usually.
There seems at least some good documentation now:
May be the FDA problems on Mojave 10.14 could be worked around by using the local, hourly Time Machine snapshots for UrBackup?
That’s a documentation of the file system itself. Not about the utils to manage it. (I don’t want to write I file system driver to create a snapshot 
).
Still no dice for tmutil https://developer.apple.com/search/?q=tmutil https://developer.apple.com/library/archive/documentation/Darwin/Reference/ManPages/man8/tmutil.8.html