A few suggestions. One is to just leave them in place and clean up around them. You can really fight with some of these things and wind up scratching or ruining something you don't want to scratch or ruin.

I have recut the screw head slot deeper with a Dremel tool and file before which gives me much greater "purchase" or grip on the screw head for breaking loose. You definitely need the correct screwdriver for the job. It has to be correct fitting and have a sharp profile so it will hold good.

The penetrating oil may take a couple days to do its job. Keep dumping that on. Sometimes some carefully applied heat can coax the screw loose but be careful with the torch around the wood and the penetrating oil if you try it. This is not highly advised but can help.

If the slot is buggered a little for the counter clockwise turning, one trick is to try to tighten the screw a tiny bit by turning it clock wise just slightly. This can be easier if the clock wise sides of the slot are not buggered up as much. This might allow you to actually break the rust loose then you can unscrew it counter clockwise.

Good luck. Others may have some more suggestions.