[Lynn Eades]

Gina, It is probably obvious to you, but may not be to some, so I will state the obvious;
The countersunk screw is (was) designed to be tightened (loosened) from the nut end. The taper is (was) supposed to sit cleanly in the countersink, and "bite into the taper. The purpose of the slot in the head is (was) to hold the screw from turning while it was still loose. I guess it can be likened to a taper on a shaft, when the two components are assembled nice and clean, they will come apart cleanly (easier than when rusted etc)
I would expect those heads to seal completely metal to metal and rust free between the faces. To me the best shot is to back the nuts off while the screws are still "stuck", and then pop them out.
I hope that helps somebody. It has worked for me, at least some times. (Carrier screws are a lot smaller)
Your various penetrants or rust removing compounds will have a better chance working in the threads of tensioned c/s screw.