I’m sorry about your mare’s mane! Where exactly is her mane being rubbed out? Is it just where the high neck ends, or is it all along the inside surface of the hood/blanket? I prefer a lower-cut blanket and a hood, and I’ve had success in the past having a wide strip of a super-slippery but not too soft fabric sewn all along the inside of the “crest” of the hood. You want something with a really tight weave – think stiff satin.
If the hood is so heavy that it rubs no matter what, try putting her in a slinky/sleazy that covers the face, neck, and shoulders. As long as it’s fitted long enough, it should really minimize the amount that the hair gets rubbed around as she moves her head and neck up and down to eat, drink, etc.
As far as getting the hair to grow back, my old standby is MTG. It stinks, but it works great!