I think you nailed a lot of good points.
I'm hesitant to comment too much because there is much we learn about Seamus in future chapters and I don't want to take away from the reading experiences by giving it all away. This is what I will say. I think if we take a step back, we can acknowledge that these behaviors, however juvenile and immature, are natural human behaviors. We've all manipulated situations to some degree to get what we want. We've all acted like children when we're scared of losing something. It can be frustrating when we're on the outside looking in, but it doesn't negate from the facts. Seamus might be older, but it doesn't mean he always gets it right. He's a flawed person just like the rest of us. He's also a growing person, just like the rest of us.