In theory that's true, but a couple months back I ran into a bug in 5.0R1 on a 630 after destroying a volume with 22 disks (on a 24 disk filer). The filer wouldn't boot off disk until I booted off floppy and ran download. I think the vol destroy command uses a "hardware erase disk" function.
Nope.
It doesn't do anything to the entire disk, it just changes the disk label to mark the disk as a spare.
I don't think we were able to reproduce it, but we may not have been following the exact same script. Did you create any volumes after destroying the volume but before trying to boot from disk? (That *does* zero the disks in the newly-created volume.)