well since U2 has it's stage we all just saw, you can be pretty sure that the $51 tkts are GA because unless U2 is going to put seats inside the oval, which would be silly, it'll be standing. Those phone people probably don't even know a thing about U2's show.
Go in person anyone that can, that's where you get the best tkts at the MGM box office, there are ladies in a box office at the front by the reservation desk, and there is a main box office back by the arena. You can also usually call up and buy tkts directly from their box office, only problem there is it will be a busy signal for hrs, but someone will get through, so it's worth doing that besides online, plus use another phone to TM. I think the seating is 15k - 20k.
Certainly hoping that U2.com is letting fanclub members, original Prop members at least, in on a presale. At this point, there is probably not many who have not used their codes.
ETS sounds like a wonderful idea.