LV Focus, sparkplug failures.

[LaserLikeFocus]

Were the plugs aftermarket and out of spec?

I had you beaut Bosch and they were running too hot. Went back to NGK copies of the MotorCraft standards.

Heat/knocking can break them down. The heat and vibration is a catalyst to particulate matter to act like a corrosive material.

You might have a claim against your mechanic or the plug manufacturer. You might have no recourse other than a new engine, cheap Focus 2nd hand or a new Corolla.