Just a small addition to your excelent little FAQ. Although I think what I'll say is more specific for Germany, could be used by any major with some modifications.
After you research the Kampfgruppe doctrine (hopefully before Barbarossa) and have that new HQ model, IIRC an HQ with Engineer brigade will have the same speed as improved tanks with self-propelled (A or AT) brigades. It would be nice to attach one HQ to an spearhead panzerkorps (under von Manstein, Hausser, Guderian or other competent leader), since it will give all HQ bonuses even when you quickly expanded deep into enemy territory, far away from your leg infantry. Plus, your HQ will hardly take much damage.
There is also no problem grupping HQs together with other spearhead korps, even leg infantry (except when you do amphibius assaults, because of the 3-divisions limit without penalty). Just consider wheather it's more useful to use it right in the advancing frontline or behind it, depending on the situation (and, of course, the number of HQs you have).