We have been there in August the past two years. Imagine walking around all day in a sauna, with the sun beating down on you. That’s about what it is. If you can get past the fact that you are going to be hot and damp with sweat all day, you will still have a great time. Umbrellas and midday AC breaks make it bearable, as well as going in and out of indoor rides/attractions during the hottest part of the day, or just taking a pool break. All that said, we did it in masks this year including my youngest kids age 3 and 7 and everyone had a great time. There was once a time I swore off WDW in summer, until I began to want to go so much I didn’t care! And, high school and football began to dictate our schedule. However, given the option, I would keep trips between Oct - April for better weather.
I would expect low crowds the end of August since school will have just started for most. As for the other questions, don’t know but I’d expect the longer you wait, the more will be back.