css hide all the comment reply links except the lowest nested comments

I believe the easiest way to do that will be with javascript. Something that will navigate at each comment list and show only last button. Something like:

jQuery(document).ready( function($){

    //Hide all buttons
    $('.comments li .reply-button').hide(0);

    //Navigate through all comments
    $('.comments li').each( function() {

        //Lets check if there is children comments in this one
        if (! ($(this).children('.children').length > 0)) {

            //Don't have, so let's show this button
            $(this).find('.reply-button').show(0);
        }

    })
})