1) When to line up if you want a good spot:
4:00. Seriously. At 10:00 there were around 200-250 people in line. At 2:00? About 25 more people than at 10:00. At 4:00? Maybe another 100-150 than at 2:00. Great spots on the floor were available for quite some time after the gates opened; probably at least a half hour. There was PLENTY of room available on the floor even once it was "full."
The GA line was so abnormal to how it has been in the past that it was simply head-scratchingly odd, in that there were so few people lining up in advance.
2) Best spots on the floor?
Anywhere in the "50 yard line" range, either against the railing by the catwalk or near the Red Zone railing. The band spent a good deal of time on the "e stage" and catwalk and fans who were near the main stage probably saw little of those parts of the show; the area near the main stage was pretty crowded compared to the rest of the floor. But be warned: if against the catwalk rail you won't be able to see the band at all when they are "in the screen." Actually the screen, period, will be of little use to you. (This is where I was... about halfway down the catwalk and against the rail; arrived around 1:30.)
3) Time doors opened?
6:15.
4) How CC entry works:
No ID was checked. In fact, my ticket taker specifically said it wasn't needed. That may be venue/ticket taker specific. Card is swiped and tickets print out. You then go to a (crowded) table and show your ticket in exchange for a wristband that directs you north or south. Be prepared to needle your way in to the wristbands folks.
All in all, best overall GA experience I've ever had at a U2 show (and I've had a lot of them). I think you could literally stroll in an hour before the show and get a great spot on the floor. Don't succumb to the temptation to be near the main stage. Lots goes on away from it, and frequently even when the band is there Bono is elsewhere. The show started and ended with a good amount of main stage action, but the middle 60% pretty much occurred elsewhere. And if you want the "big picture" be willing to back up and be nearer the stands.