Is it possible to "shim" the seat with some washers or a steel plate? Just asking.
Hubby's original seat in his truck was supposedly new, but within a year it flattened out. A quality seat is very important in your business.
Also, you might try hemorrhoid cream for the rash. It has an anti-inflammatory property to it, that should reduce some of the pain. If it works on baggy eyes, it should work on sore butt cheeks.
I'd say if a new seat is out of the question, then you might have to turn a wrench or two to resolve this problem.