Now the real problem comes out. It's not that she has sensitive skin or that she is trashing her legs while riding down the trail. But rather that you have a bug problem.
Fly spray works on certain types of flies. If you apply the spray and still have bugs bitting her legs. Then you probably are not applying a spray that works for that type of bug. And no there is no generic fly spray that will repel all types of bugs. Some bugs are just going to ignor any spray you put on.
My gelding is plagued by gnats that chew on his ventral line under his chest and belly. I've yet to find a spray that will repel them. He gets itchy from their activities and bites, rubs, scratch himself raw in that area. He does more damage than the bugs every did. While I can't replel the gnats, I can treat the symptoms of that come from the itching. Thus reducing how raw he will scratch or rub his bottom line.
Covering your horses legs while she stands around may stop or reduce the bug bites. But I'm not sure that splint boots or other types of leg protection are the most cost effective or cover the largest area. They are usually designed to protect the tendons and ligiments for horses that are performing activities that have a greater chance of the horse stepping on itself. Laterial movements, spins, sidepasses etc.
I don't live in your part of the country, So I can't offer any advice on what works for your bugs. Ask your local vet what he/she might suggest to reduce the bugs or irriation they leave.
But in general, most horses can survive the hordes of bugs that come with summer, And a few scabs are part of life.