A year ago May I did that same trip. Two weeks on Roatan then one week on AC. I just went to the Roatan airport (no reservations) and caught a flight to San Pedro Sula on Islena Air. Islena is owned by Taca. It's very similar to Tropic and Maya Island flights.
I made reservations for the SAP to BZE flight as there was only one flight a day. They might have added more since then. An overnight at Microtel Hotel and catch the early morning flight to Belize City. The hotel is a short cab ride from the airport and has a restaurant onsite.
There is a ferry from La Cieba to Punta Gorda I believe. Several local people told me it was an easy trip. It might be something I will do on future trips.
I'll be happy to give you more details if you want.