I'll try to help a fellow Floridian out. First off, most of your questions come down to your personal preferences. How badly do you want to attend law school? If you're willing to gamble on getting into FIU (and often, holds lead to rejections), then go for it. If you're dead-set on attending school next year, however, it's probably worthwhile to send in the St. Thomas fee. I may be in a similar position - waitlisted at schools I'd like to attend - so I may send the initial deposit in at one school and pray that I get in somewhere else afterwards. In addition, check into whether or not the St. Thomas deposit is binding.
Another thing that people on this board love to stress...don't count on transfering. Since everyone will be relatively smart and hard-working, there's no guarantee that you can finish high enough to transfer. You certainly could, but it shouldn't be something you plan your future around. If you attend St. Thomas, be ready to stay there for 3 years.
Finally, contact FIU and let them know your situation. If you stress to them that they are your number 1, they may be inclined to help you out. Hope this helps you...