If technique alone can address issues of head retention, it seems that using an ingredient whose sole purpose is to aid in inadequate head retention is a shortcut. While carapils may be "less dirty" than adding straight maltodextrin, neither of them feel right to me. Like I said though, it's your beer so do whatever you want with it.
As for reinheitsgebot, it has pretty much been tossed out the window by every home brewer and the vast majority of brewers worldwide, save the few whose styles are inherently "pure." We all do wheat beer, add the occasional rye, use spices ad nauseum, and almost no one bottle primes by adding speis.
Using DME to hit a missed OG is a cheat, there's no arguing that. I'd much rather let my beer go and see how it turns out than use a manufactured product, purpose-built to fix my mistake. Just me, someone who would rather drink a mistake that I made than drink a success that I used a band-aid to fix.
On Deck: Saison "Jardin d'été" (3rd Gen 3711, Wild bugs, Pale ale malt, wheat, Willamette dry hop)
Primary: Saison "Vomissure de Grenouille" (2nd Gen 3711 from dregs, Pale Ale malt, Crystals and Willamettes)
Secondary: BM45/Spontaneous Bugs Experiment (down to 1.004, and tastes awesome), contemplating what fruit to add.