Use a custom callback to render the comment content. Inside of the callback function call comment_reply_link()
wherever you need it:
comment_reply_link(
array_merge(
$args,
array (
'add_below' => 'comment-body',
'depth' => $reply_depth,
// + 1 to offer always a reply link for a consistent UI
'max_depth' => $args['max_depth'] + 1,
'before' => '<p class="reply-line">',
'after' => '</p>'
)
)
);