I see both of your points of view here, and wonder if perhaps Austen was purposefully murky on this subject. Maybe we're meant to look back after getting the shock of the Willoughby blowoff and try to figure out if he was always acting a part with her.

I don't see his agreeing with Marianne that much different than Elinor's except the self interest aspect. Maybe he just was infatuated and wanted to see her approving eye some more. The way that scene is written it's possible to imagine he could be quite taken with her. I'm just saying for example, I could imagine a person while disagreeing strongly on something, perhaps softening their answer to someone they really liked. (Interesting to note that Col. Brandon is also attracted to Marianne's eagerness, but in a more unselfish way)

He had to be somewhat conversant on the same books as Marianne, but I also wonder, if there is not more of Marianne's inconsistency here. It's possible that she is so infatuated that she is inclined to approve everything he says (whether agreeing with her or not), which is in opposition to her mantra of a lover "must enter into all my thoughts and feelings" (sorry paraphasing). As far as inconsistency: an example is that she recognized Col. Brandon's taste even though it was not rapturous delight.

We don't know what types of things they're disagreeing about either, before he concedes her point of view. Are they inconsequential or major outlooks on life?