Is it safe to cancel tmutil delete halfway through?
You have two choices: let it run and delete everything; or stop it and see what's left. The residual backup may be incomplete, in terms of files and times, but it will be something. 

My advice for TM has always been: don't mess with it. let it do its thing. When the drive gets full, either let it start deleting the oldest backups (which it will do itself), or get a bigger drive and start again or transfer the archive.