Nice. You seem to be able to control things easily in the NoticeBlock, so perhaps try by adding this to that block, right after where you added the button? If it doesn't work, the javascript part should probably be moved to the footer of the page so the reply form gets loaded first. I obviously haven't tried running this myself, but I think that the general idea could be this simple.
<script type="text/javascript">
$(".b-container")[1].hidden = true;
</script>
<br><br>
<small>I want to comment in this thread because I have something important to add - <a href="#" onclick="$('.b-container')[1].hidden = false;">Show reply form</a></small>
It's somewhat hackish, especially how I target the reply element, but I couldn't find a better identifier when taking a quick peek at the code.
(For what it's worth, the generic Post Thread you added could be made to automatically point to the same subforum as the user is in. It would take some javascript and get the relevant part from the url of the last breadcrumb.)