Honestly, you do bring up a lot of good points, points I'd never have considered. I'm not the most analytical person around, so I tend to agree with what others say about a given work. Considering most people say Conquest is garbage from a writing standpoint, osmosis will take over at that point.This is where I disagree with most people's complaint about Kamui's attitude during the Anya route. Most people have this strange idea that Kamui trust Garon, but there is nothing in the game to actually indicate this. As you mention earlier in your post, Kamui chose Anya because of his attachment to the Anya siblings. He also believe that if he can lead the Anya army with his siblings, he can stop the barbaraic acts that Anya is associated with and help them be a better nation that doesn't have to shed so much bloodshed in their conquest (this comes from his belief that their has to be good in the nation due to the kindness that he experience with his Anya siblings).
Kamui's conviction actually get challenge in the game itself at Ch. 13, where he witness the attrocity that Hans and the soldiers commit to the people of Cheve, and how his siblings do nothing to stop these barbaraic acts (out of obedience and fear of their father). In the beginning of Ch. 15, Kamui begins to question if he made the right choice in joining Anya. Then the whole Touma reveal happen in which after discovery the truth about Garon, Kamui makes the plan to reveal Garon's true form to his siblings using the Byakuya throne. Now, some people like to call this plan contrived, and question why couldn't Kamui just simply tell the siblings about Garon's true nature (ie. showing the crystal ball to them). As for that "criticism", I would simply rebutt by saying that do you honestly believe that the royal siblings, who has spent years of their lives obeying their father in his conquest out of loyalty and fear, with Marx spending that time deluding himself that his father will become better once this war end, would honestly accept his claim just like that. Absolutely not!
Therefore, Kamui takes the role of pretending to be an enforcer of Garon's will throughout the late-game of the story, while trying his best to minimize casualties in the process. For example, some people question what is the point of saving the Byakuya royal family in Ch. 18, when they can simply end the war by executing them. To that, I say do you really believe Garon is just going to leave the civilian alone once his biggest enemies (the royal family) are dealt with. No! Helping the Byakuya family in that Chapter gave Byakuya a fighting chance to protect their home, minimzing the bloodshed in the process.
There is much more that I can discuss about the story of Anya, but suffice to say, the Anya route is one of my favorite Fire Emblem stories, and I feel that so many people do not understand what exactly is the writers trying to tell with this route, and are instead viewing it based on their biases of what they think the route should be like.
That said, I think the biggest beef people have with Conquest (and If as a whole), is that Ganz, Macbeth, and Garon are such comically evil scumbags that it takes people out of the story completely. Granted, tyrants pardoning war criminals like Ganz and doing fuck all to stop their crimes aren't unheard of (I mean, just look at modern western society right now). Another point is Anya's famine only being brought up in Birthright for some reason, when it's the reason Anya goes to war.
The big issue I have with Anya's route, though, is that chapter 15 revelation that Garon is just some slime golem, and that apparently, the only way to expose that to the Anya royals... is to conquer Byakuya and hope to the Light Dragon that the Terrible Trio don't start randomly murdering everyone in sight (y'know, because Ganz is a war criminal and would absolutely do that). It is a massive risk to take that effectively guarantees Byakuya will despise Anya for possibly the rest of time and make any attempt at making peace impossible, even if any surviving Byakuya Royals happen to go along with a possible treaty. Kamui just comes off as massively oblivious to that possibility, and you know, maybe that was the point, that it's pointless and hopeless to stop a warmongering nation from doing what it does best.
However, that doesn't stop the disconnect players are going to have with this utter milquetoast, goody-two-shoes of an AVATAR character. Kamui was meant to represent the Player, so if the player doesn't want to go along with the whole "I must conquer Byakuya, killing possibly thousands of soldiers and even civilians, in order to portray Anya as having good within itself", will go over as well as Trump warring with Iran for Israel's sake ('cause the President represents the American people, I Believe this analogy works here). I still think if Mikoto and surrounding peoples weren't directly killed by Garon's hand, (and that stupid barrier plot point didn't exist) the animosity towards Kamui's decision wouldn't be as severe.
There's also the fact that Revelation completely destroys any and all sense of story and worldbuilding, so if we ignore Revelation, then I feel a lot more accepting of Anya's story.
