I haven't tested this idea, but it should work: disable SIP (reboot to recovery, from Terminal csrutil disable), delete Safary 13 (sudo rm -rf /Applications/Safari.app), reinstall macOS Mojave 10.14.6 Combo Update https://support.apple.com/kb/DL2010How do I downgrade properly?
This is one of the huge advantages APFS offers. The installer will snapshot just before doing the installation, so you can easily roll back. I've had this feature allow users to undo a failed system update more than once.I turned it off, and I also solved my problem. After pulling my hair out for over an hour I resigned myself to a wipe and reinstall. I have never backed up from TM before, but the clone I have was a few weeks ond and I didn't want to lose any data.
I booted into the Recovery Partition and selected my latest Time Machine snapshot, from only a few minutes before I closed Safari and it auto-updated. I thought it would wipe and reinstall, but I was surprised to see it only took about twenty minutes. I guess it does a progressive restore only replacing what has been changed since the snapshot/backup was taken. I didn't know TM restore worked this way.