I agree with most of the logic behind your argument, but there are two issues I don’t think anyone can ever resolve:
- B&W turned Tyrion, Daenerys, Jorah, Samwell into idiots, behaving in a manner completely inconsistent with their past. Tyrion’s bad plans have been the most obvious, but Dany completely lost her mojo, Jorah dedicated his life to get close to Dany, then volunteers for a suicide mission, and Sam decided to become a man of action, using his carbonized father’s own words. Even Jon Snow sunk to new depths of stupidity (although he didn’t have very far to fall), by not telling Dany to roast the Night King immediately. Having Sansa act smarter than her normal character arc is a huge stretch.
- Littlefinger was too smart not to read all the signs that he needed to find some pretext to flee and then make another deal with Cersei: emotionally rebuked by Sansa a number of times; confronted with evidence of Bran’s ability to see into Littlefinger’s darkest secrets; and Jon’s threat to kill him if he touched Sansa; and, seeing Arya’s fighting skills. They all were threats, and any one of the then could have ended Littlefinger’s life. Littlefinger’s strengths involved leveraging his clients’ perversions and darkest secrets, and creating conflict between already factious houses harboring a deep distrust and hatred for one another. At Winterfell, these type of circumstances did not exist with the Stark children.