In my opinion taking the time to go to"Hell" is a huge waste of time and money. Its just a tourist trap, Hell consisits of a tiny post office jam packed with tourist trying to buy postcards just so that they can be postmarked from "Hell".
The turtle farm is very interesting especially if your time is right after a hatching, those little guys are so cute! The big grandaddy turtle is a huge creature.
Haven't been to the rum factory so i can't comment on that.
I would look into an independant island tour over the ships tours on Grand Caymen. You didn't mention any interest in seeing the Sting Rays any reason? Sting Ray City/Sandbar is an amazing excursion.