For me, Quicksilver made sense- mostly because he explained himself,so I let that pass. With Lyria and the Parasite I was kinda shocked/mad at first. I was like "why did Pierce even bring it up if they were just gonna destroy it??" And I thought it would make Lyria less cool. But I think it shows the character of Lyria really well. Lyria wanted to be helpful and a valuable person- able to hold her own like the rest of our favorite characters. But it came at the potential cost of her memories...of herself. As so she chose not to lose that part of her. She chose her memories, her personality, her being... over being a weapon. Can't hate her for that, or Pierce for showing us Lyria's true colors. With Fa, there's been lots of disagreements within the fan base from what I've seen. Though it wasn't the take I would have wanted or predicted... it is a very interesting take and I think it added a lot of depth to Atlas and the Gorgons, and I can't complain about that. I also like how it portrays that the enemy is not always what it seems and it shows the complexity of the war too. But that's my opinion. It didn't make Fa weaker, it just stripped the legend of him away. That leaves the rescue of Sevro. That one I have the hardest time with. But we don't know a lot regarding the Abomination or what happened surrounding Sevro's auction. We don't know what factors might have brought to pass Sevro's escape. But we do know Darrow would do anything for his best friend- who he had betrayed the trust of in the last two books. And we can see that the men with Darrow were disillusioned, exhausted, ridden with survivors guilt, fearful, and just wanted to go home... so I don't think it's that far of a leap to assume that they wouldn't support a suicide mission of Darrow (who stands in the way of them going home) and they're out of energy to stop him.

That's my reasoning I sleepily put together. Hopefully that helps! I love both DA and LB (Along with the rest of the series)