I think you guys are both right in what you are saying. There was a lot of instability after vito went away. But I don't think the direct attacks on the rizzuto's were right away.
I may be wrong, but there was always a lot of animosity from groups outside of vito's inner circle. Those on the outside just had to go along with whatever vito said. When vito went away a lot of the animosity that always existed started to show. It was the inability of the leadership group to extinguish those fires that demonstrated an opening. It's like when the parents are away, the kids test the baby sitter. If the baby sitter can't keep control, then all hell breaks loose. Nick sr wasn't very diplomatic and arcadi/del baso/giardano tried to deal with things (like granby) through force. When other groups saw the lack of ability on behalf of the Rizzuto leadership, they saw this as an opportunity. Then to compound the problem, project colisee happened. So it was wide open for the opposition groups.
I always thought the direct attack against the rizzuto's for actual control over montreal as a whole came when del peschio was murdered.