James how many engines have you ran at .018 squish? Or under .030 for that fact. Just curious.
That plug looks terrible. Its showing signs of massive overheating. Unless the black on the base ring wipes of with no solvents. What I am seeing is the plug is running so hot the fresh mix is flashing and baking onto the base ring, don't confuse that with anytbing to do plug wise yet. Its all from too much heat in the head. The plug can't transfer heat as fast as the alum head. So its the first thing to show overheating. From what your describing your getting preignition also, this is likely the cause of the wide open issue. For the record , a 18 cc dome can't really be cut to work with a big bore engine properly.
Just for kicks cut It the threads off that plug so you can see up in side. Where the porcelean meets the steel.
As far as the head good move lapping it. But you also need to check the top of the cylinder. The ruler trick is only good for a quick obvious issue. I'm thinking the top of the cyl is not flat or that the sleeve has sunk. Slightly causing some of the head gasket issues along with too much compression and preignition.