I was in WDW from Sep 20 to Oct 8 in 2010. For about the first 1,5 weeks the air humidity was very high, but after a rainfall about 1,5 weeks out the humidity dropped. We had around 30-40 C every day I guess. All of my family walked in shirts and shorts every day all time. Can't say I felt the big drop in temperatures from day to evening (with the exemption that the sun didn't shin directly on you).
Regarding the Halloween party tickets, I'm not sure when they come up, but you can buy them at disneyworld.com
For the tips in general, I would say bring sun screen, don't stress, drink a lot of water, take rest, consider have something on your head. And you might try to avoid the transportation system at rush hour. You could also consider to rent a car to experience Sea World, Universal etc. And if you rent a car one of the first day you are there, try to go to Wal Mart or similar places to buy water to have in your room (cheaper to buy at Wal Mart than WDW). A good idea here could also to rent a small fridge for the water (and other stuff) in your room.
Feel free to ask more if you want