Cheapest option is probably via POS and PBM, least hassle would probably be the direct MIA-CAY AF flight...

AA flies to Port of Spain (POS) twice a day from Miami, one of these on a 767-300 (which would be very nice on this long flight if you can upgrade), and BA flies to Tobago (TAB) from London Gatwick (via Antigua but a direct flight so one segment only) and POS-TAB is a quick 50 mile flight. Also, flying to POS instead of FDF, you'd most likely save 2 LONE4 segments, since FDF can only be accessed from SJU and then only 4 days a week.

BWIA (BW) and Surinam Airways (PY) both fly POS-PBM, and fares start at $175 roundtrip for PY, $205 for BW. Then PY has published fares starting at $160 roundtip PBM-CAY. PY POS-CAY fares start at $474, so it seems like you'd be better off getting separate tickets.

However, the timing of all these flights is horrible... POS-PBM flights all arrive PBM at 01:00. PBM-CAY departs at 14:45 according to their website , but for some reason they don't appear on reservation systems (looks like they may be codeshares operated by a different airline). Then the return arrives PBM at 14:05, and flights to POS depart at 04:00 or 06:00. And to make things more complicated, everything on PY and BW operates only on certain days of the week...