Resolution: Portions of FMS15, SuperContainer and MirrorSync were evidently all corrupted/broken by the Security Update. I ended up doing a full uninstall of FMS 15, a complete reinstall and deployment of FMS 15.0.2 onto OS 10.11.6, followed by reinstalls of SuperContainer and MirrorSync. Everything is back up and running after 10 hours.
Lesson - even for simply described Apple security updates to an FMS server machine, create a SuperDuper clone before you start so that you can recover and not lose time rebuilding. This security update ending being rather large and involved and evidently did a bunch of stuff under the hood.