Excellent thread. I was a bit confused by the story but reading this thread clarified a lot of things for me. Thanks for this thread OP.

I chose to interfere in the final moments, because although Olgierd was a heartless character who exposed his family to complete jeopardy and loss of life, throughout the story you realize that he felt remorse for what he did. O'Dimm on the other hand, is a far more clever, dangerous threat to the people in Witcher's universe. Since the beginning his arrogance showed through his actions, how he reins people in, through his clever and evil ways and then takes their souls.

I could have avoided interference in the final act, but then what?. Olgierd never gets to completely realize his folly, O'Dimm continues to torture and deceive people for their souls. Just like he deceived Olgierd by showing him the moon on the ground and not taking him to the actual moon. Also like you said, then what did Geralt learn from all the ordeals of Borsodi, Vlodimir and Iris' quests?. In character, Geralt would not do much, just fulfill his contract, get the reward and be on his way.

But Geralt is a much more complex character than that, he's a man of trade, a trade which involves going up against demons, Olgierd would learn sooner or later but O'Dimm, he's arrogant, boastful, takes advantage of people's weaknesses, deceive's them, stops time, puts a spoon through a poor man's eye, it's him who needs to be defeated, Olgierd can't do it, he's caught in O'Dimm's snare completely.

Enter Geralt of Rivia. There is a reason Geralt isn't phased by the mark O'Dimm put on his face, there's also a reason Geralt isn't intimidated and shrugs off the time stopping gimmick of O'Dimm by just saying, 'Show off'. As an objective investigator of the whole Olgierd and O'Dimm intrigue, Geralt was the only person who could outwit O'Dimm.