In terms of getting there in the first place, BA fly from LHR-BGI, which I think has an F cabin. AA fly MIA to both BGI and POS. AA also offer 'direct' flights to/from other places, but they are very much a moveable feast and tend to go via MIA.

Getting between the 2 on an xONEx involves going via either MIA or SJU. Whichever option you take it seems a waste of 2 sectors. I'd suspect you might be better booking something on one of the local carriers.

The biggest problem at this stage is dates - the airline schedules certainly won't have been firmed up yet. And do you know what dates you want to be in each of the places - have the fixtures been arranged yet?