It would probably help desensitize her to plastic bags, tarps and other items. It will build her confidence in you and herself. Maybe it would eliminate some of the problems she is having. Then, as you said I would just slowly increase the amount of time you leave her tied. One day brush her then leave her tied for half an hour. A couple days later maybe make it an hour. It doesn't have to be everyday. It will build her patience.
"Horses don’t have hard mouths, they have hard, stiff bodies. The softer you can get the horse through his body, the softer he will be in your hands." Clinton Anderson