Your Option #2 above can work, and you'll still be able to track the ticket online after purchasing but before attaching the SM number. Instead of tracking by SM number, you'll track it by confirmation, e-ticket, or credit card number. If you look at "My Trips" area on the delta.com home page you'll see what I mean.

You mentioned she won't be flying until after the name change, so she won't have to present government ID to anyone for travel on Delta before then.

Option 3:
Call Delta now and request name change on her account due to marriage. (Personally, I'd be okay with telling them you're already married, rather than that the wedding is still pending.) Then book the ticket using the original SM account and new last name.