Your question re Sansa...
The way I see it, it is not a case of all or nothing. No single person is to blame for Ned's downfall. Sansa played a role, certainly, but it would be unfair to put all the blame on her. But it would also be unfair to exonerate her. She was not privy to all of Ned's plans regarding Stannis, the gold cloaks, etc... but she knew more than just that her father planned to spirit her and Arya away from King's Landing. She knew when they were to leave, on what ship, how many men would be in their escort, who would have the command, where Arya was that morning, etc... all of which was useful to Cersei in planning and timing her move.
Ned's talk with Littlefinger was certainly a turning point, though I am not sure I would call it =the= turning point. There were other crucial decisions that could easily have changed all had they gone differently. You mention Ned's refusal of Renly, which was equally critical. And there is Varys to consider, as well as the minor but crucial player everyone forgets -- Janos Slynt, who might have chosen just to do his duty instead of selling the gold cloaks to the highest bidder.
So... all in all, I suppose my answer would be that there is no single villain in the piece who caused it all, but rather a good half dozen players whose actions were all in part responsible for what happened.
Hope that helps.
(And let me add that I am always astonished to be reminded how fiercely some of my readers argue these points. It's gratifying to know I have readers who care so much, although if truth be told sometimes I get the scary feeling that you people know these books better than I do... )