The A, B, and C are not sections. The letters are the order you board. First, they preboard. Then, they call the A group. All the A's board and get seats. Then B and C, in turn.
That really sucks that with the package you can't get your passes til 4 hours before?? You'd think they would let you do the 24 hours, since that would be a big deterrent to the package, at least as far as I would be concerned.
I am not sure what the situation for SW is in BUF. I have flown US Air and Continental, but never SW.
I would call SW and maybe mention that you would have reconsidered buying a package if you knew you were going to possibly be last on the plane and possibly have to split your party up.
For our trip, we bought DS1 a seat for his carseat, so we will preboard and don't have to worry. You don't happen to have a small child or lap baby coming with you???