I liked your theory that he was sort of imprinting on Natasha and Steve at the time - like, maybe he wasn't all back so much as just taking out their targets the way he'd taken out Loki's for him. And maybe Natasha doesn't really have time to notice that there's still something wrong with him until afterwards, when there's not so much going on anymore - because he was acting kind of off, with Loki, but not that far off. He's still got initiative and smarts, he's not all obviously mindless drone.