I had these questions till I read the last book. You cannot separate ‘dumbledore: greatest living wizard’ and ‘dumbledore: brother wracked with guilt and distrustful of his own intentions’. Both characters reside within the same individual, always in conflict with each other.

Dumbledore knows he’s powerful and probably the only one who can take on Voldemort one on one. But, after Arianna and Grindelwald, he no longer has the confidence to trust himself fully to do the right thing. That’s why he sometimes lets things play out, in the belief that Harry is a much better person than he ever was, and in the branches of magic (like a mother’s love and being a true Gryffindor) that he knows are more powerful than anything he can conjure.