Vertical scrollbars are very handy for code blocks so you don't have to scroll all the way down the page just to get passed them. It's all there but in a handy little scrollbox.
I don't get your comment on immersion, though. A forum is a forum. It is what it is. It's not trying to be something. No disbelief to suspend.
Changing the page the forum sends you to after a reply is a simple checkbox switch, though. Although, nobody should ever ever be making second replies on a forum. Double posts are something to be avoided, not encouraged. Read the entire thread then reply and address each point. If you have more to add, edit your post. Nobody likes double or triple posters.