this engine has the 6 inch spark plug boots, so the plugs are deep in the engine, the owner broke a plug (in the back) and is now panicking over not being able to get it out. They're tried all (ok, a few) of the tool kits designed for broken plug removal, none have worked.

I've suggested removing the valve cover and spark plug boot cylinder(6 tall inch or so tube around the plug) for better access (though I'm not 100% sure the boot cylinder tube comes out (easily?))

This plug is stuck well enough that it's destroyed several (cheap) socket wrenches (currently I think there's an extractor that has finally caught and gripping)?

Would acetone be the best penetrating fluid? acetone & transmission fluid?

and thought about maybe welding something to the plug to twist it out? Yes, I know gas is explosive, but if we disconnect the fuel line, and crank the engine a few times, would that clear enough out to make it safe? AND, I know welding with the ground connected not in the immediate area (right on/by the weld point) would cause micro arcing that would destroy the engine, but is there any chance that would work?

JB Weld? I've never had a lot of success with that in high tension uses....

 

I do not believe the spark plug tube comes out, using a broken bolt extractor is probably your best bet, I am not sure how you would weld something to the broken spark plug that deep into the engine.
If this came to my dealer I would charge to remove the bank 1 head and extract the broken plug on a work bench.

 

You're right, no question, I just know there's no way they could afford to have a shop (or probably even a shade tree mechanic (btw, I'm like the Charlie Brown Christmas tree mechanic O:-) )) remove the head... And considering the position of the engine, I don't think the head could be pulled without removing the engine?

I question the spark plug tube because Im sure I've seen parts stores SELLING the tube, but I've yet to see 1 video showing how to remove the tube...

So, I'm just digging......

 

Depending on what is available where you reside, a local machine shop has tools that you probably never knew existed. You might inquire if they would want to look at the job.. What looks impossible to most of us is not that difficult with the proper tools and experience. Before making a final decision , might pay to check this out.