You are under a pavilion with fans, so it will still be warm. 2 good things though: First, it's at the end of the day, so things start to cool down a little. Second, Fort Wilderness is pretty well shaded, so the sun won't be too bad.
In my opinion, the category upgrade is not worth it. You can still get to meet all the characters, both before and during the BBQ. No need to pay extra to see them early.
The upgrade gets you seats closer to the stage. But when we went, and were at a picnic table right by the stage, I didn't like it. Personally, I would rather sit further from the stage. I had people standing all around me while I was trying to eat, taking pictures and watching their kids have fun. I felt crowded and uncomfortable.
I went up to get Mickey bars with the kids and when we came back, there were people sitting in our seats watching the performance!
As for the best time to go, if you are somebody that can't stand any sort of heat, you should probably go in late fall (November) or early spring (April). I think June and July are the hottest with the strongest sun. If the heat does not bother you, any time is great.