But considering each molecule separately, there certainly =is=
momentum change at each collsision.

True, because that's what pressure is. Ok, let's say we drop the
temperature down to absolute zero. Now there's NO momentum change.
:-) Ok, while true, we're getting away from the conditions under
which lift is generated, so it's unrealistic.

You're right that pressure itself is a momentum change, but that
doesn't appear to be what the momentum change advocates are, ah,
advocating. Now, after thinking about it for a while, I have no idea
what they mean. The idea of taking a mass of air and "throwing it
down" makes a nice, intutive image, but I can't quantify it.

However, the pressure change below the wing isn't downwash.