FYI I think the last party is Dec. 20. You said "weekend before Christmas" and that is the Friday of that weekend.
We usually go in the hot hot hot summer as well. Last year we went down in early December. We had an "unseasonably warm" weekend, which was great for us, we wore t-shirts and shorts. Others have gone in Dec and needed coats and gloves for late nights. I just watched the weather the week before we left to get an idea. We had sweatshirts and gloves but luckily never needed them.
We visited the Grand Floridian to see their decorations and gingerbread house. We went to MK in the morning, then took the monorail over there and relaxed for a bit before the Xmas party that evening. We also went over to the Yacht or Beach Club to check out the gingerbread carousel. GF might be easier with a
scooter on the monorail if you could only choose one.
My DD only wanted to do things during the party that she couldn't do regularly, so for us that was special character meets, shows, parade, fireworks.
The Candlelight Processional at Epcot was really nice. Huge choir and celebrity narrator telling the Christmas story. Epcot has Santas from around the world telling their stories but somehow we could never be in the right country at the right time.
Osborne Lights at DHS was amazing. We were there when the lights came on and I remember thinking to myself that this alone was worth the entire trip.
If you go over to the Theme Park Planning board and search
MVMCP or Candlelight Processional there are a lot of threads.
Have a great trip, it is really an awesome time of the year to go. And no mister fans needed!