Game of Thrones: Season 7: Sansa’s Motivation with Littlefinger, and a Lesson for Writers (spoilers)

I agree with your criticism. D&D have become increasingly cryptic in their writing, leaving lots of ellipses for viewers to fill in. And once Arya went to Braavos and learned secret agent ways, they’ve been especially so with her. We needed a hint or two. The upshot is that Arya behaved so bizarrely with Sansa to scare Littlefinger into making an self-incriminating mistake. She knew he was evil from Harrenhal, so she assumed Faceless Man mode to flush him out. After the PUBLIC sparring, she gave LF the 'death glare' so he knew a deadly fighter might come after him. In Ep 5 she surveilled LF and learned he had spies. Since she might be overheard, she had to be convincing and consistent in her menacing play-acting. She ensured their arguments could be overheard, but staged the last behind closed doors. Clearly expecting SAnsa's search she left the dagger and Faces easily findable. She calmly explained about the FAces, FM, her powers etc, scaring Sansa until…SILENTLY giving her HIS dagger as a signal that Arya wasn't the threat, LF was, and the ball was in Sansa's court. So Sansa told LF Arya was a Faceless Man. Realizing an FM could easily kill him, he HAD TO immediately act and insinuated that Sansa should kill Arya. BUT but for a patently false reason, which Sansa saw through. She talked to Bran (a deleted scene) and the Pack must have planned the trial.