For the Copy to Tape:
I think you must install a second Storage Daemon in the second server and make the copy/migrate jobs through Bacula, not moving the files 'manually'. You could find the info into Bacula Docs.
Restoring:
Yes, it's possible recover data with only the Volume, you must use the bscan utility from Bacula to recreate the Catalog.
Retention:
I would use a minimum of two months Retention period for recovery the data easily from local Disk (if the disk space is not critical) and use Tape Storage (on second server) only for 'archive' purposes.
All the info into Bacula Docs !
And more info:
Sorry for my rusty english...
Javier.