The Island Event that you mentioned is the only way to train a builder in v1.0 once all of the construction projects have been completed. There are only 3 huts in Village Sim, and the message about a housing shortage has been changed for v1.1 to reflect that 65 is the maximum number of villagers for most Palm models. I do know that it's possible to complete the game by training a builder on debris - it just might take awhile.