I've removed these in other boats, but not a Grady - so take this with a grain of salt. When it comes to "buried" screw heads, try looking closer for a tiny slit in the vinyl where you can access the screw head. Move/maneuver the vinyl such to try and expose that little slit/hole. You might have to push that center strip up or down to expose the hole.
Like Dennis said, mine had the phillips screws in the slots. Use a very bright LED spotlight while you spread them apart and look for that little slit, then use a new/good phillips head to find the bite into the head of the screw and back them out.