And here I thought it was a good idea to buy better plugs.If you had coolant getting into a cylinder, you would not have the black soot around the perimeter. The black soot is a good thing. Clean metal would indicate it being constantly steam cleaned from vaporized coolant. Your first picture is very blurry, not sure if it has soot or is steam cleaned.
I think you may be onto something concerning the minor debris that is seen in some pictures. Your engine treatment may have caused it which seems harmless.
One thing I question are the sparks plugs you installed. I have learned over the years to use only direct replacements. My sister's Toyota Camry had the SES light on because her certified mechanic installed an "equivalent" set of spark plugs. Soon after she sold the car to my son. I installed OEM spec'd plugs soon after her tune-up which solved the problem. I cannot take credit for the diagnosis. My next door neighbor is a mechanic and he gave me that tip.
Only one of four plugs have evidence of overheating. I suspect possible damage.
Operable word being "better". Ypu have to use the correct plug for the engine and application.
Lots of problem sourcing/solving today. My friend with a good deal more car knowledge than I came over.Hey squil....a few things that most of us have figured out here. Plugs.... Always use the OE recommended plug in the LNF motor. It doesn't like gimmicky or different plugs, unless you have a larger turbo and can prove you have spark blowout. Several have tried gimmicky plugs and gotten CELs and fixed it by putting the old plugs back in. Part two...you have a turbo and any type of stop leak is (at least I believe it is from my dealership experience) horrific in a turbo car. Its almost as bad as putting sand in your coolant for a turbo car. Not sure if you have a turbo car or not... Part 3, the rear cylinder, from what I've seen, always runs a bit hotter then the other 3. If that one plug came from #4, then that would be my thoughts. Good luck at figuring it out!