I have lowered a few focus's and they all seem to rust and bolts are impossible to get out of the rear subframe for some reason. My 20 year old cosworth ones came out easier.

The nut is only really welded on to make it easier to loosen/tighten so you dont need to hold both ends. So i dont see why you cant just get a new one and if you can get access dont even weld it on, if not just tack the new nut on. No idea on the thread size tho.