I have to admit, these are interesting observations that, in all of the LBE thread, appear to have never been raised.
As far as the lack of secondary air producing a dangerous situation, even at full throttle, as long as you have a big enough gap at the back of the stone and a sufficiently sized vent, I don't see the secondary air dropping to a dangerous level. Still, I think it's invaluable information to be aware of the fact that if you shortchange the secondary air in any way in an LBE, you're creating a dangerous situation.
The impingement aspect is interesting. Russ, when you talk about incomplete combustion, you're really referring to smoke and soot formation and not a huge loss in heat, correct? How far from the flame should objects be to avoid impingement?