I've been trying CBT for the last couple of years and my understanding of it is that B is the main reason for that approach. For people with depression, negative motivated cognition is so easy and habitual that trying to overcompensate is a good and useful strategy.

I don't think your argument for C quite works though, because those beliefs do have anticipated experiences attached to them. Example: "I am incompetent" -> I expect to fail at everything I attempt. Which then leads to the further negative belief that there's no point attempting anything since I already anticipate failing. The more helpful replacement I would choose in its place would be "I am capable of being competent if I try" -> I expect my level of effort to reflect my final results, which in turn would encourage me to work hard (well, in theory ;)