Hi David,
I was able to test out a couple of attempts to recover a collection or sub collection from a backup. Neither attempt worked. I tried opening both catalogs at the same time but Photo Mechanic will close the main catalog when you open the backup catalog. It will close the backup catalog when you try to re-open the main catalog. My second attempt was to open the backup catalog and then open the missing collection from the main catalog. Then I closed the backup catalog and opened the main catalog. I created the missing Collection and then selected the images still opened in the contact sheet, from the backup catalog. When I tried to use the Add to Collection option, Photo Mechanic popped up an error "Preloading Metadata has failed and the operation has been cancelled". It looks like the best solution is to backup more often and be careful when right-clicking on any of your Collection names.
Best regards,
--Bob