Could the sealant that comes with the heli-coil insulate the spark plug? I have heard of others using them for plugs so I would not think this was the problem. I agree that it would be much better to take the head off before doing the drilling. I do not think a valve would be down enough to be hit by the piston if there was a shaving from the drilling. It would have to be a big chunk. Have you pulled the plug out? how does it look? If you disconnect the plug wire from that plug does it change the way it runs? If so it is not getting spark or a good seal somewhere. Could you be getting blowby past the heli-coil?

Good luck

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Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)