Regarding rental car versus taxi, if you know the island well, which it sounds like you do, you wouldn't worry about getting lost and missing the ship, which is a BIG deal. It is an extra hassle though, picking up the car and turning it back in. How much actual time will you have on island? Some ships are only in port for 6 hours or so, which isn't much. I can't tell if you want to do an around the island tour or those questions were just general questions. If you have say, 10 hours in port, I probably would just rent a car. As you know, if you want to go to Happy Bay, you would have to make a time to have the driver come back and pick you up, as there are unlikely to be any taxi drivers hanging around in the parking lot. If that were me, I'd be nervous about that. Although I guess you could walk over to Friar's and get a taxi to come there.

As far as Happy Bay, it lost most of it's vegetation in Irma, so there is no shade, if that's an issue for anyone in your party. Others answered most of your other questions.