I've googled this quite extensively (I think), and I've looked everywhere in WP and Atahualpa, but I can't find anywhere that I set that I want to always use single post pages. I want to use the SEO option to make post titles H1, and that only works on single post pages.

A single post page is just that - a page with just one post. A multi post page is your blog page where you see multiple posts. You can go to dashboard->settings->reading->Blog pages show at most X posts, and set that to 1. This way you will only have one post on even your blog page.

I tried that, but my post titles are still tagged as H2, and my blog title as H1, despite having the "Make Post/Page Titles H1?" SEO option set to Yes, and max posts per page to 1.