Well then, how did they do a compression test? Can they use an adapter that doesn't screw into the spark plug hole threads?
After the compression test the OP tried to install the spark plugs and found the hole plugged up.
Unless he put his spark plug wrench into that hole BEFORE he put a spark plug in his wrench, I don't understand how that obstruction happened. Still, the helpful suggestions on how to remove, whatever it is, are good.
it will be interesting to see what it is and 'how' and 'when' it happened.
Some info definitely seems to be missing.
PS: I guess that while he was installing sparkplugs in the adjacent cyls the rubber plug may have dropped out of the socket and into the hole. I would think he would have realized that when he put the spark plug in the socket to do that cyl, as it wouldn't stay in the socket without that rubber plug. 🤯