It's always quite a struggle to keep people doing it correctly on CSS-Tricks, so I'd love if there was a plugin to make it more foolproof.
The overall idea: make it so if you use <code>
tags, it will come out perfectly.
<code><h3>whatever</h3></code>
becomes
<code><h3>Whatever</h3></code>
<code><h3>Whatever</h3></code>
stays
<code><h3>Whatever</h3></code>
<code><ul>
<li></code>
becomes
<pre><code><ul>
</li><code></pre>
<pre><code>
<ul>
<li>
</code></pre>
becomes
<pre><code><ul>
</li><code></pre>
<code>
<h1>Example
becomes
<code>
<h1>Example</h1>
</code>
Just a thought, but what about creating a shortcode function and instructing your commenters to use that? That way you'd have complete control over what happens inside it without having to modify how the other portions of the comments act.