Dettlaff is an idiot and way too impulsive, even Regis told you that Rhen did abandon him, in the first place, something he did not wanted to belive.
Honestely, i was in Dettlaff side, UNTIL he attacked the city, there is just no excuse for that, what you are saying about Syanna can be applied to Dettlaff as well, and thats the whole point in the history, but, you can talk Syanna out of it if you bother to investigate her side of the history, Dettlaff will not listen to reason, and even attack Geralt and Regis, also for not reason at all. He got what he deserved, he may be not evil, but being that impulsive is just as bad.
Also Syanna just wants revenge against his sister, the killing of the 4 knights was to conceal his true intentions, and the reason of why Anna will suddently be murdered. Syanna wants to kill his sister for the wrong reasons, that why Geralt is able to talk her out of it, and that you only are able to if you learn all the details.
And thats the only reason, i dont think Syanna wants the power either, if so, she whould not attempt to kill her sisterim plain view of everyone, and knowing that she will shortly follow.
Well, we're all entitled to see it our own way, guess that's the point of the choices.
I got all the story on Syanna but it didn't change my opinion that her actions were unforgivable. I don't think there's an option where she gets executed so it's either suicide or Dettlaff. I didn't like her and I didn't think much of AH either; she knew from early on Syanna was involved but was willing to risk the entire city.
Also, why the heck does everything happen outside?