When you cancel the award reservation you can either refund the taxes/fees from the award reservation or hold them for future use.
I would likely pick the hold funds for future use option since you are going to be almost immediately using those same funds for your taxes/fees on the new companion ticket.
As for your side note - the redemption rate between US and Non-US destinations I feel is the same (70 points per dollar on WGA). And if you were to go book tickets on another airline for the same Caribbean destination you would end up paying the same amount of taxes and fees (Customs, TSA, and Other fees).