You did it! It worked. Amazing. That stopper had been stuck so hard for so long and resisted so many attempts to free it. I did as suggested and put it in the freezer. When I took it out it didn't come loose so I did as you also suggested: ran warm water over the frozen lid. And out it glided, as if by magic. The magic of #AskFedi. This impossible task went so well that I think I will now install Linux as an encore.
Please accept this bunch of flowers as a thank-you for your help & knowledge.