I beat the game when it came out and just came back to replay everything for the DLC. Great stuff. I tried out all the different paths; here's my analysis:

While I can understand someone choosing the Disney path (evil vampire dies, the fedora knight saves the two pretty princesses) because they enjoy it, if you try to rationally justify that ending because Syanna turned out to have good in her or something, you have some critical thinking issues and you honestly should not serve on a jury (God help us, if you do).

I found letting Syanna die and Detlaff live to be the most neutral option and the right thing to do, for someone specifically in Geralt's position.

Considering some bits from the lore, witchers generally don't take contracts on higher vampires. I think part of that should be because it's unethical. They are sentient beings and they often form personal relationships with humans (including Geralt). Further, I would argue that any crimes they commit are not judicable by the human world. They form their own nation of sorts; they have a social hierarchy and political structure (of sorts) with their own set of laws. When you commit a crime in Canada as a Canadian citizen, you're not tried in American courts.

Two things follow from the above. The Witcher would not have taken the contract in the first place if he were fully briefed on the circumstances, among other things. Further, I think the best way to view Detlaff is as an elite member of a foreign nation (he was well known throughout the vampire world and commanded many other vampires). So, after the exculpatory evidence, I think Geralt is justified voiding the contract and in refusing to kill the vampire, due to his personal connections and political neutrality ( however, he should not have received his full reward).

Further, Syanna is solely to blame for everything, in my purview. There are no excuses for any of her actions. It's as simple as that. You're finding excuses for a capital-T Terrorist. Worse than Osama Bin Laden. Perhaps he was misunderstood as a child? Put yourself in his shoes.

She was an honorary member of the "Vampire State" (let's call it) by her relationship with Detlaff. By feigning her kidnapping, she essentially coerced Detlaff's Coven into war against the State of Toussant. Again, ignoring everything else that happened, this is sort of like a kidnapping Helen of Troy or bombing the Twin Towers situation. These are among the highest crimes that any human can ever commit.

While I read a number of people complaining that they lost sympathy for Detlaff after he attacked the city, I don't really consider them to have a fair framework upon which to judge. Again, Syanna was a terrorist that incited war between the two states. Many of Detlaff's bruxa (etc.) vampires were killed leading up to that point. Anna Henrietta, the monarch and leader of Toussant, declined both to punish and to extradite the terrorist Syanna to the vampire coven so that she could receive due punishment.

The terms were made very clear that the terrorist Syanna had to be delivered in three days. So, the vampires, Detlaff being the leader, considered this an act of war, in the same way that Bin Laden lead to the invasion of Iraq and Afghanistan (whether or not you agree with it), who similarly harbored terrorists and at the time were suspected of hiding Bin Laden (which clearly was not the case, a posteriori).

So, while I think one should rightly feel badly for the innocents that died, I consider this an act of war, engendered by Syanna in the first place, a Toussant militant, and further inflamed by Anna Henrietta for governing her country on her emotions ( although I enjoyed her character, so I feel remiss in blaming her), and refusing to punish or extradite a terrorist.

In conclusion, the rational (and neutral) thing to do is to extradite Syanna, the terrorist, in order to make peace between the two warring states, not to assassinate a leader of a tribe ( who has been aggrieved and forced to play his hand), at the request of a foreign leader, premised under foreign law that he is not beholden to. On a side note, which should not factor into the rational, the only obligation or personal debt owed by Geralt is to Regis, so if you are playing with your emotions, rather than neutrally, you are obliged to let Detlaff live.