You make some good points. Obviously, it is important to look critically at the research. Even if 12-20 sessions proves sufficient to alleviate the average depression, this would be a full course of treatment only if the client has no other problems which are significant enough to need treatment. In real life, most clients present with a variety of problems, some of which respond to treatment quickly and some of which can be quite difficult. Cognitive Therapy is known as a short-term approach to therapy but I would not expect a complex client like Kathy to be done with therapy after 12-20 sessions.
However, if a Control-Mastery approach takes 96 sessions to address guilt which seemed fairly obvious at the outset of treatment, it would be useful to know if a different treatment approach would produce the same result more quickly. Unfortunately, that is a difficult question to answer.