I think it's somewhere in the middle, and mainly B's using this conversation to Mean-Girl Serena into cosigning B's relationship with N as a real life grownup reality, which she's not explicitly yet done. The old I'm Going To Fuck My Boyfriend & You Will Facilitate This In Some Token Way trick. Which we've all used from time to time, if we're being honest. "Well, that's really romantic, Blair." Serena offers the possibility of her not going anywhere near this doomed tryst and ruining it by sheer dint of -- her Serena-ness? Her Sereniquity? Her Serenity -- but B's not having it. If Blair fucked Nate in the woods and Serena wasn't there to see it, what exactly would be the point? See: "No, I want you to come. In fact, I was hoping you would be one of my ladies. Would you give Nate the last clue?" Already did that, sweetheart, but I admire the balls-outness of this. It's so Blair to prove that everything's fine by violently making sure that everything is not fine. But if the plan works, then it really will be, and that's so Blair too. She's like a very violent mathematician. "Solve for X? I'll fucking set X on fire." But she really does mean all of it at the same time, because that's how people work: "Tonight is all about starting over." Her voice, and her face, get harder and more willful with every word: "I trust you. And him." And because S knows as well as we do that this is how Blair gets shit done, S gives in completely, literally bowing down to her: "Well, then I will be honored to serve you, my queen." Besides, since N can hardly be trusted, S is bringing Dan anyhow, right?