Some horses change color all their lives. The roaning is quite common in your horse's breed and is to be expected.
I applaud you on your efforts to do the best for her. As any owner should do. It's OK to worry. You only worry because you love them. When you quit worrying about your animals, then your best to get rid of them because your love for animals is no longer there.
In regard to her rain rot - I washed her with betadine and scrubbed her with a curry comb. All the dead skin and scabs came off, she looked nasty the first couple of days but since then her hair has already grown in.
I haven't seen any white hairs in her tail, or on her legs or face but she does have some in her mane. No other white hairs or marks anywhere else.