You're getting closer on the valid HTML. Now you've missing the </div> that's required for <div id="navSuppWrapper">.

I see the issue now; you've apparently either got a down-level template-override version (i.e. /includes/templates/responsive_sheffield_blue/templates/tpl_checkout_one_confirmation_default.php) or didn't install the v1.0.4 version of the plugin's file into the template_default folder.

The fact that the form's name is still 'checkout_one' and there's no associated id= attribute is what's telling me that!

If you're not making changes to the page's layout, there's no need to replicate the page's template into your active template's folders ... but if you do, you'll need to re-merge any changes to those template files each time you upgrade the One-Page Checkout plugin.