I have all the tech stuff I can get, but I cannot figure out how to get the builders to build more huts or to convert the one everyone "thinks could be converted". Am I thereby limited to 90 islanders?

Unless you have twins or triplets when population is 89. The absolute highest population is 92, unless I am mistaken.

90 is the limit, unless as said you get a twin or triplet on #89. There is no more huts after you build the three. They just wander around worrying about housing. The big hut can be converted to a school by a master XXXX, (you figure it out) if it doesn't show converted after you do it, you have a older version of the game and must go back to where you bought it and upgrade.