I can't answer the question about the crowds, but I can help with the weather.
1. It is very hot in Fl in September. Temperatures during the day would still be in the 90s unless it is unseasonably cool.
2. The usual rainy season is in spring. (This year we didn't have one and I have to keep all magnifying glasses away from my lawn to prevent a fire of epic proportions

).
Fl rain usually is torrential but lasts only a short time. With your extended stay you might be subject to a day or two with some rain, but usually it comes and blows over within a couple of hours. The best bet is to hit some indoor shows for a while. Unless a system is in the area (depression, tropical storm, or the h-word). Then it could rain your entire stay and September is still in the season, but I wouldn't worry about it too much. I know we're going to be okay this year:animwink: If not-maybe you could send me some info on real estate in England.
3. Some people take off Fri for labor day weekend, so the higher crowds would probably be for four days.
Have fun!:wave: