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A lean mixture is a prime cause high cylinder temperatures, which can affect the head/head gasket, but spark plugs don't have anything to do with fuel mixture.
They are a part of the cooling system though. Heat is transferred through the spark plug to the head. A loose plug can interrupt the flow of heat out of the cylinder. But I would expect the plug to overheat and get toasted long before any head or head gasket problem developed.
After 10,000 miles I wouldn't feel bad about the supposed plug torque issue. I'd say if the plugs were tight enough to cool themselves and last 10000 miles they wouldn't have an effect on the head.
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