Kulga what's a slide-in panel?
That panel with the discussion list that slides in when you move the pointer to the left edge of the browser window. (There's probably a more official name for it.) I think it's kind of cool how you can pin it there so you can switch topics without going back to the index page or clicking on a tag, but I've never gotten in the habit of using it.
Your idea does sound more fluid, and it would be easy to format it to blend in with the Flarum UX; but it would mean you'd be either chatting or reading posts. Putting chat in the slide-in panel (whatever it's called) would allow you to do both at once. And it's designed so you can pin the chat there when you want it, and hide it when you don't need it. I thought of putting the chat there because it accomplishes the same thing as your first mockup without adding a third column to the layout.
Both ways offer advantages. I do like your idea of putting the chat where the posts are. You'd merely have to add some sort of indicator to the Chat link in the sidebar to indicate when there are new chat messages and mentions. Gitter's sidebar could serve as an inspiration for that.
