I was going to guess that.  Honest, I was :-).  But just one correction:  The high exhaust temperature with one ignition off isn't due to "incomplete fuel burn."  The burn is virtually complete whether running on one spark plug or two.  But the burn RATE is much different.  With two plugs there are two flame fronts and the burn completes sooner.  

True

The slower burn rate on one plug means that a significant portion of the burn happens during the expansion stroke and less heat is converted to power.  

Uh, not really.  You can actually end up with MORE power, depending on where the thetaPP started.

 Further, when the mixture is leaned past the best power (or maybe peak EGT) the flame travel is even slower and consequently the exhaust temperature will likely just continue to go up when leaning, not peak and go down.

NO!  EGT will always rise to peak, then fall when LOP.  ALWAYS.  Unless, of course, Sir Isaac Newton was wrong.