cyl 7 misfire and coolant in the plug well

My '11 Crown vic with 270k miles suddenly started running rough mostly at the lower end of the gears. By the time you got into o/d the whole car shook. Scan tool gives me P0307 cyl 7 misfire, and P0316 misfire upon startup. So I pull the coil on 7 to find a pool of burnt smelling coolant about 2" deep. Siphoned and blew it out and removed the plug to find it's electrodes fouled with a thin coat of something shiny and black. Rubbed off without much effort. The plugs are about 4 weeks and 1500 miles old. Pulled cyl 6 coil right next to it and found a lot of white particles about half the size of rice stuck to the sides of the plug well walls. Seemed odd as I'd just blown out all the plug wells a month ago. Blew it out again and removed the plug to reveal electrodes that were coated slightly white. This plug had a ring of a black just south of the hex part. Looked like darkened sort of coagulated oil. Wiped off easily.

The car drives much better now after blowing the antifreeze out of the plug well but the miss is still there. So glad as this might buy me some time to get this figured out.

I'm not even sure how it's possible for coolant to get in a plug well. Any ideas, head gasket maybe? What's my next step in investigating?