Are people who did the mission arcs the first time able to do them again or is it limited to 1 time per account? (as in you finish it if you started it but have to skip what you finished)

They are available to do once. If you completed part them while the story was live, you should be able to continue where you left off.

Thanks for the info :>. That means that there is no way you can get left behind like in some games.