Here's what I'd try. I think the pic on the left is what he is talking about. Sorry I don't have a spare spark plug, so I just substituted a long bolt, but you get the idea.
I'd get some foam double sided sticky tape. You can get it most anywhere. It comes in two flavors. One is real thin like scotch tape, but the one you want is thicker, and has a foam layer between the sticky parts.
Get a spare socket about the same diameter as the spark plug socket. Then cut a couple of pieces as shown in the center pic, and stick the foam tape to the spare socket. Then drop the spare socket with the tape on it, onto the spark plug socket that is stuck in the hole. If the stuck socket isn't too oily, you should be able to get it out. You might have to try a couple of times to get it, but the tape is pretty strong, and you might be able to get it on the first try.
Let us know if it works.
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