Because the pots that you have are locked up, you have nothing to lose if you try to take them apart. I do not know if you have pots that may be easily disassembled or not. Many pots have little metal tabs that may be bent to remove the back cover. If they are frozen inside it may be best to replace them even if you can (open and) repair them.
I agree that a dual 5K should work. If you can find it, a "linear taper" is the proper type to replace it with. You may only be able to find "audio taper" and those will work well but the adjustment will change its rate as you turn them. Good luck, and, great troubleshooting job. I believe that you have found the source of trouble.
@mescalero thanks for the encouragement. Unfortunately it is riveted. I try to open it but I'm sure to break it. I also think 5K is fine but where do I buy these stacked pots? Only found 500K or 250K audio tapers