an AK is probably the most parsimonious explanation for 'how did Harry just kill an enchanted adult troll in a few seconds?'

I doubt it. Quirrell knows that, with high probability, no one taught Harry AK. There are other ways Harry could know it, for example from his memories of Lily and Voldemort using it, or through the link, or by remembering Quirrell doing it in Azkaban, but this seems like a stretch. On the other hand, Quirrell knows that Harry can be extremely inventive and extremely deadly when he means to. Also, I don't see why Eliezer would need this twist for the rest of the plot.