re: "...I think I found something else. It must be that particular sparkplug wire or something I missed...."

I don't think so -- the wire wouldn't "pull in" the fluid like that. I'll bet some previous owner or mechanic working on your car cross-threaded and damaged the threads -- it's an aluminum head, and it's soft. That wouldn't necessarily (depending on how serious the damage is) keep you from threading in the plugs, but it could make them "stiff" (just as you describe). On my cars, for examples, I can turn them all the way in (and out) with my fingers, except for the fraction of a rotation when the washer compresses -- that's the way it should be. And the damaged threads might make a pathway into the cylinder, in order to suck in the fluid.

On my cars, I always use antiseize, and I only torque the plugs using two fingers on the ratchet handle -- never more than that!