I was there in February. At that time Aero Caribe, Lhansa and Sosa where flying to Guanaja. Aero Caribe has a 6:30 departing flight to La Ceiba that would give you enough time to catch a flight or ferry to Roatan. There is no regular scheduled flights between the islands. I would have your hotel make any flight reservations, it is very challenging to figure out the flight schedules and the airline's website information is not accurate.
From Mangrove Bight there is a ferry (Ms. May) that makes daily (I think) round trips to Bonacca. That is your best bet for groceries, there is limited supplies available in Mangrove Bight. You can stop at any of the hotel/resorts (G&G Clearwater, Kays Place, Bo Bush's) on the north side for dinner and drinks, recommend calling first to let them know you are coming. There is also a local restaurant and disco in Mangrove Bight.