Is the plug fouled with lead deposit or oil?
Has the oil consumption returned to normal?
I'd be suspicious about the spark on that plug, timing has been mentioned but I'd have to ask why there's no issue with the other cylinder (no. 3?) on that side. Is it possible the plug lead is faulty giving a weak spark?
Keep at them, it's their job to fix it. I'm not sure how the insurance company will see their role in this. That may depend on the broker and or theloss adjustor involved.
At some point a fresh set of eyes may be the best option. Some mechanics become fixated on what they think the problem is and ignore other possibilities.