Well, I'd say there's a clear difference between ambushing (deception in tactical combat situations) and lying/manipulating (social deception in micro-situations).

The first requires way less self-deception: the requirement here is not control of vocal tone, facial expression and knowledge innuendo and social graces; no, here an understanding of which parts of the enemy forces the enemy appreciates, and which targets he would like to hit in your own army. The second calculations; terrain and so on are also logical advantages. So in effect it can easily be a silence-based deception yet Harry is still surprisingly mediocre in this aspect. Given that they are already in military outfits, a well-constructed ambush should be able to drop more than third of an enemy force before they even knew what hit them (this just by the most simple solution: half hidden, half baiting). A little instigated chaos by the non-hiding part might very well be necessary so as to negate the counter-ambush advantage of the maps.

About the "drilling people" together; that has already been mostly done by his reputation and being in a situation very much like The Robber's Cave scenario. All he had to do was exploit the chaos he loves to create and have his running troops run so as to V around two sides of the enemy's O positions (V and O are here used as visual representations of the formations in question).

I concur with you on the second part and I applaud the sharp observation on the aspect of self-deception/mucking one's own cognition when it comes to social interactions.