Any one of your theories as to how the #1 hole got moisture in it sounds logical.
The reason I suggested moving the plug to a different cylinder was that just in case the plug WAS the culprit, moving it to #4 for instance would have caused a P0304 code. That's why I said don't move it to the same cylinder that you move the coil to - so you could differentiate between which part, coil or plug, was the problem.
It may just be that moisture in the #1 hole had caused a bad connection between the coil and spark plug.
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