wild bill -- folks more knowledgable than myself are on this board, but as one who recently stayed North on AC I would recommend doing biking and golf carts only for fun and not the primary means of getting from here to there on AC. I know you asked specifically about land transportation but that is one heck of a bumpy and uncomfortable ride into town from up there. As for groceries, check out www.lagniappe-belize.com
and get yourself fully stocked before arrival -- no sense lugging groceries all over the island when you're on vacation. Mosquitos, my god yes, but bearable if you stay near the beach at dawn and dusk and bring plenty of repellant.