I agree with most of what you say, though I don’t agree that the letter is flimsy; it has so far been corroborated in certain details multiple times, and hasn’t yet been contradicted.
As I’ve said elsewhere, however, I think it would be a bad thing if the Pope resigns over what we’ve heard so far. In fact there is very little that the Pope could do that would make me want him to resign.
I too think Pope Francis has been bad for the Church, and I believe it will take decades to clean up the mess he’s made of theology and the Church’s teaching authority (and no, I don’t think he is a heretic; also, I tend to agree with the Pope on most matters, I just think he has done an exceptionally poor job at expressing and reinforcing unchanging Church teaching, which is his primary role). If Pope’s can be pushed to resign at the first appearance of a scandal, however, then we aren’t far from the days of poisoning Popes to replace them.
If the Pope is guilty of what he has been accused of, I’d personally be happy with public exposure and changes being made in the way the Vatican operates. We would see the Pope for what he is as a leader, and give him only the regard due to his position. If the accusations are not true then he deserves to be publically exonerated, but hopefully changes in the Vatican could still be made.