I'm not that familiar with campdrafting so I'm not sure of the rules for showing and how the judges want you to appear. But I would think you'd want a natural flowing mane, like you see in cutting horses. Check with the association you want to compete in and find out if there are preferences from the judges.
True - sort of, they usually hog their manes (cut it all off). However I don't believe there is a rule about it, so you don't have to do it. They do it a lot in polocrosse too, and often ASH's have their manes hogged even when not competing.