Jaime's singular motivation the entire series has been Cersei. It's because of her that he did his grimiest shit. That hasn't changed; he's going to Dorne for Cersei, not Myrcella.

As for Bronn, he's given the promise of a better girl and castle for his help. This is entirely consistent with his character, especially when it's revealed Lollys is not directly in line to live in the castle.

> There's no reason for it to even be secret. Send a 'diplomatic' envoy and then steal her back.

Again, it reeks to me like you've been reading spoilers and have this idea of how the show *should* go instead of how it's actually going, and ignoring the evidence placed in front of you.