I've never been when the park closes that early, as we prefer summer hours with Fantasmic at 9 & 10:30.
We've attended the 10:30 showing several times and have never arrived more than 10 minutes before the scheduled start time. Also, we've rarely seen the stadium more than half full for the late show during our visits over the past few years.
This summer on a Star Wars weekend, we arrived about 10:20 as usual and found great seats just to the right of center about half way up. There was enough room available that my kids were able to lay down on the benches until the show started.
It's a really good show, I hope you enjoy it.