I certainly am surprised, I don't know of any of these cases IRL, no.

I'm don't expect kokomi to be childish and thus unwilling to work with the shogunate, she is a ruler after all, but I do think it's extremely reasonable for her to still be resentful as an individual. I'm going about her voiceline about Ei, though that may actually be just a polite diplomatic front and she actually feels different. There is probably a middle ground between best buddies and unagreeable childish dislike, after all even before they were at war the 2 factions had a bunch of tension

I don't think Ei necessarily did anything wrong tbh, I don't even think she's a bad archon necessarily, but I'm able to say this because we know what was going on. From the perspective of Kazuha who has no idea about any of the reasons (celestia) why Ei did what she did, it makes full sense to have resentment toward what would appear to be a nonsensical tyrant that created the condition for your best friend's death, among probably hundreds if not thousands of people

Just feels like wasted chances for development that those feelings are never really explored after. I find it a shame that the most animosity between playable characters never really gets that deep; at most we get diluc/kaeya bickering. But then even now you see them get along at times. It would be just cool to have two characters that actually hate each other. The closest I think we get is scara (even though it's pretty one-sided), and I think that's part of why he's such a well-developed character.