I was thinking about this during a recent rewatch of the episode - I find myself feeling like there is a slight disconnect with the political atmosphere to this episode.
Let's put aside for a second the unfavorable light the Durass Family is portrayed in throughout all of Star Trek history (which really is just convenient heels, after a fashion for given plots). Lets think about what they were actually trying to accomplish: In this 2 part episode they wanted to create a condition where the Klingon Empire was on friendly terms with BOTH the Federation and the Romulan Empires. They may have been going about this in a very duplicitous way, but after a fashion, it had the potential to bring peace between the three waring factions.
Gowran didn't exactly have a reputation for being exceedingly honorable. He didn't have the taint of his father being a traitor, but in the long run, his actions were just as bad if not worse then Durass's father. That's not to suggest the characters could peer into the future and see his actions any less then the writers likely had any idea they would take Gowran in DS9, but certainly, even by the time Redemption played, we knew Gowran was no where near a saint or even remotely close to the leader the Klingon empire needed.
If the Durass Family would have been made out to hate the Federation more or were trying to manipulate the Federation, a lot of the events would have made more sense, but after a fashion, the Durass sisters made it pretty clear they didn't want the Federation as enemies and probably weren't going to make a move against breaking the treaty. That means, after a fashion, if they would have won, though it would have been extremely tenuous, it would have laid the foundation for a Federation / Klingon / Romulan Alliance of sorts if the Federation would have played their cards better; the Federation didn't necessarily need to violate their principles to do it.