It is a good idea to use the penetrating oil from both the top and bottom of the clamp tower hole that holds the clamp shaft. What brand of penetrating oil are you using?
At this point, don't worry too much about a replacement clamp. After the clamp is removed, clean up the rust on the clamp shaft using 120 grit (or finer grit) sandpaper coated with the same penetrating oil. You will not remove enough material to cause a concern. Clean it up enough until the clamp shaft fits and turns freely in the clamp base tower. Clean the shaft thoroughly with the same penetrating oil to remove the sandpaper grit particles and then apply a light coat of grease to the shaft. Clean the base tower hole as well and the clamp should work OK. I use wheel bearing grease, but other non-hardening grease types will work also. Check with you local auto store.
If needed, I can post a photo of a home made tool that uses the same 120 grit sand paper to hone the inside diameter of the tower hole.