The whole second half of the story disappointed me a lot, to be honest.
I just feel that the writing wasn't as good as the rest of the story. There are two things in particular that disappointed me (and triggered the final events): a) Annarietta's attitude regarding her sister and b) Dettlaff's massacre at Beauclair. Those two things were asspulls, in my opinion, aimed to drive the story towards a simpler ending.
a) Throughout the story, we see Annarietta as a courageous, severe and even intelligent ruler. At least, that's what she seemed to me. A duchess concerned about her people and her duchy, as she demonstrated while investigating alongside Geralt. Then, her sister appears and she suddenly becomes a childish, whimsical princess, unable to accept the truth and punish Syanna as she deserved, leading to a massacre in Beauclair... In my opinion, that's not coherent at all with what we had seen of her. I just can't imagine the Annarietta of the first half of the story acting like that.
b) I don't think the slaughter that Dettlaff triggered was coherent either, and Regis' explanation wasn't satisfying at all. Yes, maybe he was emotionally unstable, but we also know that he hated killing innocent people. Damn, he even cut his own hand in self-hatred and culpability after killing ONE guy while he was being blackmailed. Taking into account how he was portrayed along the story, I don't think he would massacre innocent people like that.
Leaving that aside, I chose the same ending as OP., and let Dettlaff go (even though I think Geralt would have killed him). I had a lot of trouble trying to decide Dettlaff's fate, because on one hand I think he deserves punishment for Beauclair's massacre, and on the other hand he was being blackmailed and above all I didn't want Regis to kill his friend (it was obvious since only a Higher Vampire can kill another Higher Vampire), and then have him hounded by his own species...
It annoys me too (a lot) that they decided to skip three days just like that. Yeah, let's wait until Dettlaff sends a legion of vampires to kill everyone! Maybe he was just kidding! Why should we try to stop him! *sigh* Makes no sense whatsoever.