But because Revan intervened, the events did happen. In order to prevent this from happening the Council should have intervened as soon as the Republic was threatened, in order to fulfill their promise to the Republic,
and prevent the brutal Mandalorian Crusade that was already threatening the Republic (Revan intervened because he saw this was happening, he effectively saved them from destruction). If Revan had never intervened, the Mandalorians would have swept forward, forcing the Jedi to fight them as finally they themselves became threatened. The result would likely have been the fall of the Republic, and if not that, they would have fallen to the Empire soon after. Hence irony of their actions.
No it would not likely have been the case. The Republic was still a ways from falling. And if the Jedi have intervened later, it would definitely not have fallen if they had defeated the Mandalorians. If they had intervened earlier it would have been worse in the long run. There was a reason The Emperor manipulated the Mandalorians into attacking first. But otherwise there wouldn't have been any of the deaths that followed in Jedi Civil War or the Sith Civil War, so the Republic would have been stronger if Revan had listened to the Council. If the Republic didn't fall from those two conflicts that directly followed the Mandalorian Wars, then it wouldn't have fallen if the Mandalorian Wars had lasted longer.
As for them not being responsible for the actions of Revan, they were. The Jedi are responsible for the actions of all their members, even those who leave the order. Take the discussion between members of the Jedi Council after the exile of Meetra Surik:
As you can see the Council held themselves responsible for Revan's actions. They (apart from Atris - but she was a fanatic and fell to the dark side) would have disagreed with you. And while they may not admit it (Zez-Kai Ell admits it later) it was also their apathy that led to Revan's actions. They are responsible. They let loose a dangerous force to wage war across the galaxy, one that would inevitably lead to the dark side and the repetition of the events of the Great Sith War. Would the same thing have happened anyway? Even under the Council's guidance. No. Revan had the power to manipulate, but ultimately he was just a Jedi, and had no power over the Council's decisions. Their decision would have been final, and the answer to the mass shadow generator would have been no. But it would never have come to this, with the full backing of the Jedi - the victory against the Mandalorians would be a decisive one.
The Council wad not apathetic, they were overcautious. And you're absolutely underestimating Revan's drive to get his own way. He was manipulative enough to be willing to let his own allies (sometimes entire worlds) perish if it benefited him in the long run. He would have found a way to use the Mass Shadow Generator. And even if there had been surviving Council Members that told him no. He could have simply defied them, like he did to go to war in the first place. With so many more Jedi embroiled in the Mandalorian Wars he could well have ended up with more Jedi turning.
Blaming the Council for Revan's actions makes no sense and absurdly absolves Revan of any personal responsibility for the horrors he caused. Revan was effectively a traitor to the Jedi Order. It's not the fault of the people he betrayed that he did what he did. Revan made the choices, he alone bears responsibility for being "a dangerous force that waged war across the galaxy". Just because they didn't stop him and his followers by force doesn't mean they are responsible for everything he did.
Obi-Wan blamed himself for the fall of Anakin Skywalker but it still was, at the end of the day, Anakan's fault.