At first glance it seems like a @grant issue, replace the script header with this:
// ==UserScript==
// @name YouTube: Auto-Load More Comments + Toggle Comments
// @version 8.0
// @include http://www.youtube.com/watch*
// @include https://www.youtube.com/watch*
// @grant GM_setValue
// @grant GM_getValue
// ==/UserScript==
YouTube: Auto-Load More Comments
There's a script for this at userscripts-mirror.org already, but it doesn't work anymore, and you can't log in there to contact the author; he/she doesn't seem to be registered here, or at least not under the name they used there. I figured I'd mention it here, perhaps someone can do whatever minor modification is necessary to make this work again.
There seem to be no end of ways to completely remove YT comments, but this is the only thing I've found that will load more. Any help would be really appreciated!