Good summary above.
> Gandi should have system level backups to get their VMs/systems back up and running again
Note that the VMs did not go down. It's just a storage unit that crashed. From talking to an affected user, their VM is still running happily, except that it cannot "see" the data disk.
> Either way, using ZFS arrays and relying on ZFS Snaps to restore from isn't the best strategy, ZFS is great until it goes wrong and then it goes really wrong
I have no experience with ZFS but from their communications, that is indeed the problem they're facing at the moment with attempting data recovery.
Agreed with the rest of your points, especially good point about the restore plan. It is like in aviation, where landing is your plan B; your plan A is going around. It puts you in the right mindset.