How did she have a choice in being sold off to Ramsay?
Exactly.
Also the knights of the vale showed up because she sent that letter. I'm not sure why you're trying to take that away from her as a character. The show literally spells it out in the narrative.
I explained why. LF likely would've come anyway since he'd already decided to bring the knights up to Winterfell
before he knew anything about Sansa or Jon's plans.
Sansa writing that letter gives him a reason to legitimately be there, but he was there with his force ready to attack anyway.
That's not taking away from Sansa. That's what was happening. This wasn't a shrewd military move by Sansa.
And again, I feel like I'm repeating myself: Littlefinger is an EXCEPTIONAL manipulator. Sansa hasn't had a father figure, or anyone who gives a shit about her, in her life since her father died. Except Littlefinger. He preys on her insecurities. He convinced her the marriage to Ramsay would be alright because he'd convinced her for so long he had her best interests at heart. Basically going "lol what a dumbass she got manipulated!" is exceptionally ungenerous considering like, her entire life since the start of the series.
Yes, and that's why I don't see her as having a particularly strong character arc. Does she have a good reason for plotting against Jon?
Nope.
She's jealous. That's the sole motivation. A motivation planted in her mind by Littlefinger to help him gain the north. If she actually does that, she's the same petty girl that wanted to marry Joffrey. Not an astute player of the game since she'll have gotten nothing she wants and given LF everything he wanted.
And Jon himself said "You're the lady of Winterfell", so again, you're arguing against what the show is directly saying? Lots of bad faith stuff here.
Bad faith? Come on now.
Try to examine that sequence from Jon's perspective. He didn't know if he'd have any support at all going in to that feast. He certainly wasn't expecting them to wholeheartedly accept him or for them to rally to him as their king.
He was gobsmakced. He looked to Sansa and she smiled and nodded. At what point would him standing up and saying "That's nice and all, but Sansa is smarter than me because she wrote a letter and kept it secret from everyone!" have been a grand idea?
Also Sansa said he should be lord of Winterfell first. They were trading niceties.