December, especially after the 15th or so is considered high season for many places including resort areas, south/central america, Mexico and Caribbean. Even though the price is high, if capacity does not increase, the price may not come down by much.
You could look at some cheaper options: Jetblue? not sure if Spirit flies from FLL? there are also some low-fare mexican airlines (Mexicana/Volaris).

B fare is technically not an automatic upgrade, only if there is availability. Your wife, if no status, will waitlist at the airport as a Gold companion (would not bet on her getting the upgrade)

Obviously you CANNOT sit in the front but you could let her and the baby sit in the front and you get yourself a nice exit row seat. That is the right thing to do if you want to start your vacation on the right foot. Better yet, you sit with the baby in the back and let her relax and have a couple of drinks in the front of the plane.