My second impression, I guess, was that Sins of Sinister is trying to be both focused very tightly on a small handful of key characters, while also being massively broad, universal, in scope such that it touches absolutely everyone. When this story was being relayed purely through the perspectives of Destiny, and Sinister himself, that was completely ok. Any "but what about..." questions could be dismissed as not important enough for our narrators to mention.
But now that it's taking over Immortal X-Men for three months, and is crossing over with Red and Legion, I think there's a more obvious tension with trying to actually fit the cast of that book into a story that wasn't really made for all of them.
We're told that the four compromised Quiet Council members - Charles, Bennet, Hope, Emma - are still the same people they have always been. It's early, and anything could change or be recontextualized later, but their own impression seems to be that their memories and personalities are fully intact, and Sinister is not controlling them. So it seems to be some kind chemical change at work, altering their disposition or inhibitions, and possibly they're not as inclined to resist if Sinister is gently nudging them along. And he seems to confirm this himself, that he left them with free will and their personalities in place, though later issues could always say he misjudged, and less of their real selves survive than they like to think.
But if that's all true, it seems like things escalated very quickly to them being on board with his world domination plot. With Charles, okay, fair enough, he's been susceptible to this kind of thing before. And Hope is still new to all of this, and Exodus is an unreformed supervillain whose goals have only coincidentally become aligned with the X-Men (though I would have thought he'd still think Sinister's plan is stupid as hell, and just leave him in the Pit).
But Emma has already dealt, many times, with intrusive malevolent forces taking control, or seeking to hollow her out from within. Emma is already well accustomed to acting in opposition to the impulses of her worst self. She has a support system, that she sought out herself over many years, to check her if she starts to backslide.
(Without even getting into any of the weird metaphysical stuff at play: she's a psychic, she has a hive mind with her daughters, at least half of the time her brain is made of diamond)
I would have liked to see her's be a slow, gradual descent, if she needed to go through this at all, worn down as everything crumbles around her anyway. But as it stands, the story seems like it just needs her to be evil right now in order to work.
It's unfortunately a very clumsy way of capping off her journey from where she began in the Dark Phoenix Saga, to where she's come in recent years, all of which should inform how she responds to being infected. And it's particularly jarring because of the clash in tone from issue to issue. When Sinister assassinated her, it was presented as slapstick. Exploding Xavier brains! Evil communion wafers! The art carried it, and the idiocy of that chain of events could be excused because it was there to make readers laugh at his clownery.
But Immortal X-Men no. 9 is no longer just an irreverent comedy, it is now also the inciting incident that transforms Emma into a genocidal evil queen. I do not really believe that those two concepts are suitable for grafting together within the same story. I suppose anything is open to good execution, but this transition happened very quickly. It doesn't seem to have taken more than a few hours within the story.
That's why I think the story is trying to be much too broad for it's focus. I would have liked to take more time with only Xavier, or only Hope, at first and chart a slow conquest. Secondary characters like Emma could either be gradually worn down, as part of the background world building, or could just peace out from the story entirely to give more space to the more central figures.
And when we introduce so many side characters, it begs too much clarification on what is actually going on with each of them. Sinister implies that it was the Quiet Council alone who kept their personalities, but some of them needed to present themselves in public and pass scrutiny. Could the the Council have changed the plan once they were in charge of infecting others, and loosened up control? And was it the whole Council who were excluded, or just the original four? Were Illyana, Hank, and Namor included when they joined? There's also a question of who Charles is just outright mind controlling, or perhaps editing with Cerebro. And every character at play has their own idiosyncrasies to their personal circumstances.
I would assume based on what we know right now, and knowing that we only have three months to get through a complete story, that only the original four both kept their personalities, and also escaped being constantly controlled by Charles. Because otherwise it gets weird. Colossus is already being mind controlled by his brother, who would not be in favor of Sinister ruling Earth, so I think that would fall apart if he'd been left with any autonomy. Namor had to manage to pass as himself to Doom, but also needed to not kill Charles and take over as soon as he was done. Magik has dealt with being Darkchylde for most of her life, and it seems not-very likely that being a Sinister would be harder to resist and overcome than being the Hellgod of Limbo was. (I would say the same for Kate and all the weird brainwashing and corruption stuff in her history, but Immortal X-Men's contempt for her is evident enough that I think we're just meant to assume she was killed and mind controlled immediately, to cut down on needing to write her. I will say though that it's already making for a very weird reading experience, having X-Treme X-Men overlap completely with Sins of Sinister as scheduled)
It could and should be elaborated on in the future, because knowing whether or not people like Magik, Exodus, or Namor have free will and are still open to reason is vital to understanding the stakes of the conflict. But having rocketed past the first ten years of Sinister's conquest in just a single issue, instead of drawing it out more, I think this inevitably feels very rushed one way or another. This could be a matter of personal preference, but I think it's always better to just exclude characters from a story, if they would get in the way of telling it, than to alter them so that they behave as it needs.
And it's not a bad story! I do recommend it as a continuation of what was already the best Destiny story we've ever had. And it's setting up Ororo to have a very good story of her own in the three Brotherhood issues. But it's a shame that so many other characters are only there to be there. I think it compares a bit unfavorably with Judgment Day, which was just very well motivated. Even if you didn't want to read about Marvel Earth bring judged by one of its gods, every character, and not only the main stars, got to behave as their own problems and social pressures dictated.