People tend to confuse Cushing's and Insulin resistance, because Cushing's horses so often have insulin resistance. The hoof issues you are discussing are from insulin resistance, but many horses with Cushing's do not have insulin resistance so don't have hoof issues. My Cushing's mare does not have hoof issues or insulin resistance.
Insulin resistance causes the abnormal fat deposits and laminitis.
Cushing's alone has different symptoms. They include the long and/or abnormally shedding hair coat, muscle loss along the topline, pot belly appearance, sweating abnormalities (either too much or too little), drinking a lot, urinating a lot, lack of energy. The biggest issue, however is a tendency to get infections easily. Those might include thrush, abscesses, pneumonia, or any wound infection.
Most likely your horse has Cushing's. Probably that is why she is lacking muscle along the topline rather than due to lack of exercise. My mare had some muscle loss along the topline in her mid-20s and gained it back without exercising after starting on the medication for Cushing's.
Most likely your horse's Cushing's is progressing so slowly that at her age you wouldn't necessarily need to put her on medication. If you're fine with clipping her, that's probably your biggest issue it sounds like. However, just understand that her cortisol levels are high and depressing her immune system, so any infection could easily get out of hand and be the end of her. But at 28 I feel that is a given for horses anyway, even without Cushing's.
I really like how my horse does on the Prascend, she sheds out, is able to sweat normally, has a great topline, and lots of energy for 28. However, to give 1 mg. (1 pill) a day costs about $60 a month.