Re: Crowds in the summer??
From June to Mid-August, the crowds are heavy. The only busier times of the year are Easter Week and Christmas Week.
We were there for July 4th week in 1998, and it was super hot (the wildfires were burning), and quite crowded, plus Brazilian tour groups were all over the place. As long as you use a touring plan and FastPass, you'll be able to see most of what you want.