Hi grpaine.
Welcome to the Norton Community Forums. 
Interesting questions! 
1 - Yes. I believe that is the point of the backup, to restore baked up files onto a restored or replaced computer.
2- Yes I believe you can. But why do you not try this yourself? As I understand it you have three machines on a network. Try restoring a file from one of the three onto one of the others. No need to even do the restore just find the file and get ready to do the restore.
3- You probably could if you wished but I would want to try it first before going firm on that. Personally I would just set up a second backup set to alternately backup to the other drive. Then you would have two independent backups.
Did that answer your questions and did it help?