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);
}
})
})