Another plot point I'm not wholely clear about.
I am curious as to why Sansa Stark backed Little Finger rather than outing him to the Veil nobles.
Can someone with the knowledge drop a spoiler for me?
This was originally a puzzling deviation from the book for me, but then I thought about it.
As I recall from the book, Sansa played along with the whole 'Alaine Rivers' deception Petyr was plotting for her. The nobles investigating Lysa's death never found out who she really was. He told her it was to protect her from other enemies, although we know that he's playing some kind of long game that revolves around his keeping Sansa in his custody. Sansa suspects something might be up, but she isn't sure and she also knows that Petyr is the one that got her out of King's Landing. She plays along until she knows more, but she still isn't sure who she can trust. Too snakebitten by all the shit that happened to her to this point, but she's growing.
In the show, if I had to guess Sansa's motivations, the reason she spills the beans about who she really is and then protects Littlefinger with the lie about Lysa's death is because she's simultaneously tired of hiding her true identity but also knows that if she tells the truth about Littlefinger murdering Lysa, then those nobles take the Vale over and she's just a pawn being shuffled around. Ultimately, by revealing herself to them and still protecting Petyr, she sets herself up more as an equal to him, someone he now can't harm or touch without the suspicion overwhelming him. Lysa's death could be an accident, but Sansa's? And after she gave them a version of events? He wouldn't escape the scrutiny another time.
Also, because Sansa is there and The Vale was allied with Winterfell because of Jon Arryn and his relationship with Ned and Robert when they were kids (they both grew up in the Vale), they're going to be sympathetic to her and hide her from the Lannisters. Letting everyone know she's there means they either have to give her up when the Lannies come calling or they have to go to war, and neither outcome is going to be desirable.
All in all, Sansa gambled and played a dangerous card and it was a pretty shrewd political move. She's learned from the intrigues at King's Landing.
In my view, and here's the ultimate answer to our question Mags, she did it because by allying herself with Littlefinger in the current circumstances and also making it difficult for him to simply use her as a pawn in his own game, she is able to exert a little power and have some authority about what happens to her next. With these other nobles, she has no idea what happens to her. With Littlefinger, she believes she can be more involved with what happens to her next.
The change doesn't significantly change the events of the story, as I imagine the pieces will all go where they're still supposed to go, and the secrecy allows the knowledge from ever becoming public, which would otherwise significantly change the way the story unfolds. These would be seismic disturbances to the original story, and I don't believe (again, just speculation) the showrunners are going to deviate all that much. They know it works, and they seem to be loyal to Martin's books.
From a creative standpoint, my guess is that the showrunners understand that television audiences would probably have just gotten frustrated with Sansa all over again if she hadn't grown at all from her experiences in King's Landing. This way, she can be a more aggressive character and show growth. In a book, you can show inner growth and strength without having to make bold moves because you're in the character's head. But to TV audiences, continuing to tell that same story without at least a LITTLE strength emerging might just frustrate them. I think it was a good change, given the difference in pacing between TV shows and novels.