...and of course the witches are already smarter than the garden-variety Burned Ones--they can speak, and they can exercise at least some misdirection ("help me..."). It remains unclear whether the witch that attacked MC was just in the process of turning (as MC believed), or whether that's (as suggested upthread by others) a look she can assume to lure victims.
I'm not entirely certain if this mimicry is actual intelligence. Burned Ones seem male-coded to me, and Witches as female-coded. The Burned Ones use physical prowess, while the Witches use manipulation. I don't think it's actual intelligence, because there doesn't seem to be intent behind their behavior. The males are slow, so they prefer to catch targets unaware (hunter behavior). The females, physically weaker, benefit from targets coming to them. And even their shape seems to be
designed to appeal to male/human instinct. Maybe their words are the human's last thoughts of despair.
It remains unclear whether the witch that attacked MC was just in the process of turning
You're right, this isn't entirely clear yet, though we have seen witches who were completely turned already. We just don't know whether they can turn back or not. This also makes every woman we'll meet a potential danger. Other groups/people could also think our women are witches if encountered alone. Just imagine Laura breaking down and banging on a door, screaming to be let in... How easy would it be for people inside to think she's a Witch?
One technical error that's bugging me is the effect of A/C on the fog. I can't speak with certainty about commercial A/C, but residential A/C in the United States recirculates air (through a filter); it doesn't take it in from outside. So "those houses have had the A/C running" not only wouldn't bring in fog, it would (slowly, because those filters usually aren't all that effective) filter it out.
You're right, certainly if they're HVAC systems. To be fair, I don't know that much about AC/HVAC, but I do suspect some fresh air needs to be let in, even if it's just to replenish the oxygen in an environment. Also, if the filters are made of a kind of plastic or fiber, it's possible the fog would melt/eat them over time, like the high-dosage exposure did with clothes.