Wow, that was a long post xD
Anyway, I read over it and I still don't see how you guys don't get what David is doing and how he's doing it :O
First off, David has no much idea about Dante and Isabella at first - he thought Dante and Isabella were siblings (vitriolic yes, siblings but still . . .) So he's bluffing to Isabella about having figure things out to psyche her. Most of this info is from watching their interactions, but he was only certain of anything once he'd talked to Dante and got information from him. Once he had that, don't you think common sense would put everything into place? Jasmine wasn't particularly secretive in the beginning . . .
As for David talking to Isabella like that, that's just his character; he honestly believes that Isabella isn't as bad as Dante is making her out to be and wants to trust her; it's on his freaking character sheet. Even if it's potentially dangerous and a stupidly idiotic bad move, he's decided to just get on with it and try to talk to her, also, David at least worded it in a way that makes it sound like he just put things together by himself and not with Dante's help, so at best, he's a third-party wild card.
Come on, it's an RP and I'm just sticking to the character I made, roll with it xD