I can understand that you don't want to categorize the characters in terms of whether they liked to fuck or be fucked, and that wasn't really my intent. I struggled to come up with terms to describe the distinction that I see between the two groups besides "top" and "bottom." But I DO think there is one.
Other labels I thought of included masculine/feminine, agressive/passive, dominant/submissive.... I was trying to get at who tends to take the upper hand in their relationships, who's more or less controlling, who's more or less vulnerable.
Michael doesn't move from Brian to Ben. He moves from Brian to DAVID to Ben.
David is also a strong, dominant personality. He may not be as much of an idealist as Ben is, but he isn't anywhere near as dark as Brian is. So David functions as the same character "type" as Ben does. If we replaced Ben with David in the diagrams, we would still see many of the same patterns.
The limits of this model are probably similar to the limits I see with the Myers-Briggs. For instance, I am neither an extrovert or an introvert; I am somewhere in between. Perhaps Ben and Blake both lie closer to the middle of one or more of the axes. When we first meet Blake, he is a very dark, weak character. As he grows, he becomes stronger and more positive. When we first meet Ben, he is pretty positive about life; events along the way darken him. So Ben and Blake do become more like each other over time.