That should be fine to remove the least amount of soil. I would cut off only what has real bad mildew. Some of your inner leaves look ok, you want to get a good root system before winter.
Until it is in the ground with your full sun and happy it will wilt. Age and in ground planting will stop the wilting. After transplanting it will wilt for awhile, keep watered. Probably with problems you have had I would water around it. Normally nothing bothers the leaves. A little care now will equal many years of enjoyment. When we were first putting in the gardens we forgot about it. It was mowed twice. I do not know how to winterize them, but every spring before flowering lift all the leaves. You just might find a baby or two. You can pot them for root development, and they will cry and wilt after you put them in the ground. Any time it is ungainly give it a haircut. I pretty much take alot of mine off about now so I will have a good fall show. I root alot of the cuttings in moist soil in the shade. With those cuttings being old and the Texas heat, I still get most to root.
A little fertilizer increases growth and amount of blooms. Many blessings for a good day!
