Some reactions for you after having been in the RedZone in Louisville last night:
I arrived around 6 for a 6:30 scheduled opening act. The rail was taken, but I was second row, middle of the tree immediately behind the people on the rail. A few beers later I was a few rows further back! (But still it was only 5-6 deep.) I'm sure in bigger cities people may arrive earlier for the RedZone, but at least in some cities you can get very close without arriving way early. (It may have made a difference that it was a Friday night - maybe people arrive later on week nights than weekends?)
They don't pack the RedZone completely full. The back of it is pretty open so people can come in and out easily. Plus there's a bar back there for part of the night that takes up space. I suppose whether it's half full or 2/3 or 3/4 might vary from city to city, but I have to believe for safety reasons it's never going to be a can of sardines.
While the main stage is very high, the B-stage is very low. I'm guessing 5'. So there's no concern about the angle no matter how close you are to the B-stage.
As someone said earlier in this thread, the boom is a mild annoyance. But my memory is that it wasn't used until Exit and then for the encore. It wasn't used at all for the opening four songs on the B-stage.
The dedicated bar closes before U2 takes the stage. Get your drinks early!
The check-in process was excellent and better than on prior tours. They gave each group a personal guide to escort them to the RedZone from the stadium entrance and to explain basic things (e.g. nearest restrooms, dedicated bar) on the way there.
Don't sweat it - it'll be great no matter where you end up. And you can move around within the RedZone relatively easily.