You can back up with the friction sway attached... You cannot turn too sharply, forward or reverse has the potential to cause issues. Also, you can use either one or two for longer trailers. I have a 30' and just use one, although I own and installed the components for two. The first couple trips proved to be great with just the one. I did not have much sway, just the odd time and more noticeable when being passed by vehicles traveling the other direction.
I would recommend first making sure the current hitch setup is correct, head height and angle can have a profound impact. Second, load properly, keep enough weight on the tongue. Last, adding on sway control as needed. There is a balance of safety and comfort. Not saying to be less safe to be more comfortable, the opposite. Being safe is not always as comfortable or involves driving slower, pulling over, adjusting other behaviours.