I had this problem w/a VW Rabbit once. I battled with the retaining screw using a screwdriver but couldn’t get it to budge. I figured out eventually that to remove the retaining screw I had to use a manual impact driver gadget. I don’t know what this tool is called, but it’s a miracle when you need it and it’s inexpensive (<$20), I got mine at Sears I think. You simply configure it w/ a Phillips drive head, match it up with the screw head, and whap it a good one w/a hammer, which causes it to twist 1/4 turn. That worked like a champ. One or two whacks and the screw was loose as a goose.
I can’t imagine someone w/25 years experience would forget to remove the retaining screw, but I suppose it’s possible. It’s good that the parts folks are putting some threaded removing holes in their rotors, but I suppose this car belonging to the OP’er doens’t have that feature.
Back to the Rabbit. The retaining screw was out but the rotor on the Rabbit was still stuck. Rusted on steady and not moving a twiddle. I pulled and I pried and I whacked it with a hammer from behind. No go. So I put some Liquid Wrench on the places it looked like it was stuck, let it soak a couple hours, and brought out the propane torch. I think I told this story here before, but what happened next was rather humorous – if you happened to be a third party watching me do it that is. After alternately heating the rotor with the torch and whacking w/a hammer for 15 minutes or so, I was taking a little break laying under the car wondering what to do next. All of a sudden the rotor simply fell off! Not only wasn’t I whacking it, I wasn’t even touching it! It just up and fell off! The downside issince I was laying under the rotor, it fell right onto my leg. It was particularly unfortunate that I was wearing short pants! Whap! My bare leg! Hot! Ouch! lol
Anyway, that’s how I got the stuck rotor off the Rabbit. And to remind me of this event, I had the outlines of it sort of tatoo’d on my leg for several weeks!