Well the car has been at the dealer for two days now. They can't figure it out, and have enlisted the help of the Subaru Tech Line.
The light does not immediately come on at a certain temperature. It will either come on after idling for a couple minutes, or in the middle of leisurely driving. In one case, the coolant is cold, in the other it's hot.
I will check the #1 cylinder plug when I get the car back. Regarding the multiple posts...when ever there is a noteworthy update (a new starting-off point, per se), it seems more convenient for others if I create a new post with a nice, short summary right up front summing up all the previous conclusions so that new observers and those (like yourself) offering assistance may get to the point quickly, without having to read through several back-and-forths.
Thanks again all!