There is no way you would find the missing electrode. . If you are turning 6000 RPM, the piston is going up and down 100 times a second. Even if you shut it off right away, it has either blown out through a valve or is imbedded in the soft aluminum of the head or piston.

Yes, do a compression check first. Then a leak down if you get bad results.
It could be you have a weak injector causing a lean condition.

 

dont worry about looking for that piece of spark plug...you melted it. I did the same thing on the race track, except I did it to three plugs; and three pistons went with them. your going to be rebuilding your motor, best case scenario its only a burn't valve. Good luck.

 

interesting, stock iridiums shouldnt need gapping in the first place. your signature shows fairly standard mods, who did your tune? maybe your EGT was through the roof.