E is too broad in scope for this argument. This argument is specifically addressing one claim and why it rests on a fuzzy distinction. The author is arguing against this specific claim and is not making any statements about other similar claims. Choice A is in scope, Choice E is out of scope. The word often is what puts it out of scope, but if you removed the word often, it would still not be the main point, it would simply be a premise in support of the main point. Hope that helps.

You are correct. That is exactly what is being compared.

A vs B should be "People who read the label" vs "People who don't read it"

The quality being compared between those two people is the percentage of their calories coming from fat.

The winner is the people who read the label.

He definitely made this one a lot more complicated than it needed to be, IMO.