Cersei's plan doesn't make a lick of sense to me.
All they wanted her to do was agree to a truce and not attack while they were fighting the undead.
She refused that, and then came back and gave an even bigger pledge, to actually fight alongside them.
But she was lying, and was really going to sit it out and let everyone kill each other.
Why not just agree to the first deal then, which was basically what she is going to do anyway??
Now, they're going to know instantly that she is betraying them, because obviously her armies are not going to go north.
Also, Littlefinger not denying everything, not bringing up that he saved Sansa's life when he got her out of King's Landing, saved her life and Jon's life and the entire North when he brought the knights of the Vale to win the Battle of the Bastards...just so stupid.