This depends first and foremost on how you did the backup. Did you do a mailbox backup, or will you being restoring a full mailbox store as a no loss restore?
I assume you did a mailbox level backup so here are the answers:
1) No, just select to redirect the mailbox to the new one. The GUID won't match since you created a new account and mailbox. You don't have to delete the mailbox, but if you do you can have BE use the option "automatically recreate users account and mailbox". This is the preferred option since it will put the original record/account back into active directory. Sometimes you will have to reset the user permission level so the user can access the account, but that may or may not happen for you. Just check rights when you are done restoring.
2) In either case from above, if you did the mailbox backup then the restore does NOT take the database offline, so this is not a concern. If the user has their mailbox open, they will see messages appearing as they are restored. No other users are affected. (If you are doing a database restore, then everyone in the sae mailbox store will be offline at this time, unless you use a Recovery Storage Group in Exchange 2003)
3) As stated above, a restore from a mailbox level backup does not affect anyone else. As noted, a database restore will cause all mailboxes to be offline during the restore, so no email could be sent by anyone in that mailbox store. If you have multiple storage groups, then the users in other storage groups would be able to continue sending mail as expected. The mail will be received by users who are offline as soon as your restore finished and you mounted the mailbox store again.
Does that help?
