Missing div tag bumping sidebar to bottom

Well, you can probably narrow it down to one of the theme templates in your child theme.

One thing I sometimes do when developing themes with lots of custom templates is simply write the name of the file somewhere in the html, like right after the header tag like so:

<?php get_header();?>
single.php
<div class="container">
(etc)

Do that for everything in your child theme and you’ll eventually find it.