I agree it's unlikely that Delta will add a second flight until next December. I would call them and go to ATL a day earlier. I frequently do this now and stay at the airport Westin. The shuttle is free and runs very frequently (plus I stay for free on points).

Call sooner than later because the flights are filling quickly with only 1 flight although sometimes seats open closer to the date if they change aircraft. I've done 3 trips since Irma and 2 trips were on 757s and only the trip flight was a 737.

Another options that might work is to have Delta change your tickets to go to San Juan. There are several flight options from SJU to Beef Island.

Regards,
Jason