The first line in function showreplies hides those posts which are replies to previous posts, and the second line opens up the replies below the relevant posts. (@baris This was my attempt before I read your answer. Thanks for the pointer to the code! I will try that out).
I added the handler for action:posts.loaded in order toEliminate Reply and Quote buttons for the replies in order not to generate higher levels of nesting (this is ok for me) After adding a reply to a post, by default it is also added at the end - in order to avoid that I set it to hidden. This does not work so well as the page gets scrolled to the top after that.. - I am not quite sure how to fix it. If I figure it out I'll make an update..