I do the CPS change without disconnecting the battery. There is no power to it with the key off.

When I have to deal with a rusted bolt, I usually get out the vice-grips. Soak the bolt good with a penetrating oil (liquid wrench or like) and let it set for a while. Then clamp the vice-grips on it and break it free. There are also some nifty wrenches out now that are supposed to be able to grab these flaking bolts. I havent tried them. Good luck...