1) when restoring a VM from tape (3 month old backup) - how can the backup operator know which LUN she should restore ? the VM might have been in any one of the LUNs at that time ?
SMVI stores its own backup catalog (which is searchable) & even if you delete associated snapshot, the catalog entry stays in place. Of course if you actually try to restore from a non existent snapshot, it will generate a nice error "could not find current snapshot"). But this way you will know in what datastore was the VM in question during backup.
2) If a LUN is restored from tape, and then mounted by the vSphere cluster, will SMVI be able to restore a single file from one of the VMs in this LUN ?
I am not quite sure whether SMVI 2.0 Single File Restore will function in this circumstances, but you can always do that 'old school' way - restore snapshot, mount it to an ESX host (with option 'allow resignatures') and use a recovery VM to mount VMDK file in question & pull out a single file from it.