How do I force a certain page to use a certain single.html file?

[pointyfar]

@bluedogranch

So for some reason, {{ define "main" }} and {{ end }} are breaking.

As mentioned above,

@calebjasik

You have closing </div> tag floating outside of any defining blocks.

your contact.html layout page has the closing </div> outside the define block. This is breaking your template. See doc: Base templates and blocks | Hugo

Not Ok:

{{ define "main" }}
... 
{{ end }}

</div>

Ok:

{{ define "main" }}
... 
</div>
{{ end }}