It would seem by its very nature that usually a situation that requires “dan l’kav zchus” is one where the benefit of the doubt assumption is unlikely. Otherwise, it wouldn’t be a challenge to believe the more favorable account, as it would be somewhat plausible. In my opinion, where a favorable explanation is significantly less plausible is exactly the time where we should strive to be dan l’kav zchus. As far as I know (and I could be wrong), the requirement is not conditioned by “unless you really really think you know what’s really going on.” It’s all part of the test.
Just my view.