First of all you cannot be too bothered about backups if you can go 8 months without them 
Second is that when you enable the "backup_with_snapshots" switch it means that you don't want those snapshots anymore.
Thirdly, I have just done precisely the same upgrade from 5.0 to 5.1 without problems so if it nuked your datastore then you would have lost all your VMs as well as your ghetto install.
You do have major issues but the blame does not lie with the software.