Thank you everyone for your replies. We now have a somewhat awkward workaround that will get us down and back without a San Juan layover. Turned out the problem was that Spirit had deleted our connecting flight from FLL to BWI on the return leg from their schedule and, because of that, cancelled our whole trip. Since Spirit does not do multi-city reservations, we used the cancelled flight credit to book the BWI to SXM leg as a one way trip; added a second one way return on Spirit from SXM to FLL; and, finally, a third one way ticket on JetBlue from FLL to DCA. This required about four hours on the computer and a call to the credit card company which was convinced that buying six one way tickets was evidence of fraud. The kids have been drafted to do the drop off in Baltimore and pick up in Alexandria, so, with fingers crossed, we will be on our way in nine days.