1.Mine butted, I assume U channel is the same.
2.end pieces?
3.caulk or glue won't hurt anything.
4.From what I remember there are only 3 steel plants that make these arches in N. America and they are all pretty much interchangeable. There are no marking at all on my arches. It's kinda funny that there are dozens of dealers all saying that theirs is better than the competitions.
Do you have a bunch of sections or "L" channel....2"x8' or so? This "L" strapping goes on the outside of the building horizontally(middle, top and middle). You install it as you build your arches and it keep them in line and square. They are detailed in my building thread, so make sure you put them in...they will keep you from pulling your hair out halfway down the build.