That sounds rotten. What do you mean by "lost all application and hardware data"? Do you mean that on the MNT node you no longer could issue commands on the ADE-OS CLI? Are you saying that performing the Context Visibility Reset process trashed one (or both) of your MNT nodes? Obviously this is not normal or expected and something buggy got in the way. My first suspicion in these cases is VM corruption due to potential snapshotting or historical technical debt from too many upgrades. But since you're on SNS appliances I can rest assured there was no VM snapshot involved. And contrary to Cisco recommendation about using 3655 or 3695 as MNT is not a guarantee for success. The same bugs are baked into all flavours of the product.
I'd say the TAC would not be wrong to suggest rebuilds, because most of them don't have a clue how to fix something like this. The trouble with a rebuild is that the node registration copies the primary admin main database to the newly built node - if there is corruption in there, then there is potential for issues in future. To eradicate such issues, you'd need to build a new PAN, and put all the config back by hand (you can import Devices/Groups, Users, Endpoints). You could do this slowly over time, and then migrate your PSNs over to the new PAN. A lot of work for sure.
