They don't always form the perfect curve.
I used what I called 'spanners'.
https://www.backyardchickens.com/articles/aarts-hoop-coop-chicken-tractor.72211/
Depends on how shallow the arch is.
And what kind of panels.....cattle panels are much less strong than hog panels due to larger rod spacing.
Yes. They don't make vinyl coated livestock panel(to my knowledge).