It wasn’t a fake Sansa, it was a fake Arya. The fake Arya was the role played by Jeyne Poole, one of Sansa’s friends who was taken into Littlefinger’s ‘protection’ after her father was killed along with the rest of Ned’s forces in King’s Landing. She was trained in whorehouses to survive because she is an unimportant commoner whose father’s crime was loyalty to his lord.
Part of the point of the fake Arya plot is that legitimacy is a sham, what matters is what people believe.
It’s not just about events in the plot, there are things like themes and allegory that are ignored for the sake of epic moments like Sansa siccing dogs onto Ramsay.
There’s plenty of stuff that absolutely matters. Someone who read books 1 and 2 got hints about Walder Frey but the Red Wedding in book 3 is still a shocker. The Tyrells plotting to kill Joffrey is a shocker. Littlefinger orchestrating the murder of Jon Arryn is a shocker. There were hints for all those shocks in the earlier books, there were prophecies and visions that foretold the events that would define the series.
I honestly believe books 4 and 5 are just as good as the first three. I get disappointed when Martin doesn’t announce he’s done, but the only real issue with the 4th and 5th is how far apart they’ve been from the pace of release that the earlier ones had. I read them in order nonstop (got in after the first season of the show) and I couldn’t stop, the books are great as they are.