I set up a test disaster and this is what I discovered. I pretended that my computer was destroyed, my backups unreadable, and the only recourse was my one archive disk stored offsite. I attached my offsite archive disk to my laptop, double clicked on the LR_photos.lrcat file and found it corrupted. The image files along with the related XMP sidecar files were however intact. I created a new Lightroom Library, went to FILE and selected Import From Disk. Selected Add Photos To Catalog Without Moving. After the import, the images were present, all editing, flags, keywords were present. Only the History list and the Collections were missing. I was able to RESET an image to its original condition when I first imported it in the original library.
No, the disaster did not happen, but if it does, I know how to fix the worst senario. One more thing-- I now have 2 archive disks stored offsite in 2 different locations. Thank you Seth and Michael.