1) 4 digits is fine, but you will need to use the routing tables to manipulate it to match your extensions. For instance, if your DID range is 1200 through 1239, and you want to match that to extensions 100 through 139, you would strip off the first 2 digits and add a 1. So 1200 becomes 100, 1201 becomes 101, etc.
You can also receive totally oddball digits, strip all 4, and replace with 3 digits to hit an extension. So those 12 existing numbers may need 12 routing table entries to take care of.
For XXX-5964, you CAN'T renumber 115 to 964, because the 9 conflicts with 9 for ARS. So if you receive 5964, absorb 4 digits, add 115, it ends up ringing Ext. 115.
2) What kind of voicemail do you have. Merlin Messaging, you go to 2 for mailboxes, dial the extension number, then it's 1 to assign a mailbox, 2 for Transfer Only permission, and 3 to assign to an auto attendant. Next you press 1, 2, 3, or 4 to select which AA will answer.
Merlin Mail, you assign Class Of Service 15 for AA1, COS 16 for AA2, COS 3 for AA4
Audix, you create the subscriber as an auto attendant
3) An extension must exhaust all Primary and Secondary coverage paths before the Group Coverage path will take effect. So if 115 goes into DND, all of the extensions with Primary Coverage will still ring. Once they finish ringing for the coverage, it will cover to voice mail. You'd have to ask an AT&T engineer why they designed it that way.
Remember that it's all NUMBER BASED, not line based. You can use the Night Service button to switch the greeting from Day to Night, but not to direct the calls to the AA. That will only happen when a call to an extension covers to VM and the mailbox is pointed to the AA.