Issue #2 turned out to be what broke OPC. If you want to use yourstore + OPC be prepared for mucho CSS hacking, but beyond that, the key fix I added is in includes/classes/order_total.php.

Code:
--- a/includes/classes/order_total.php
+++ b/includes/classes/order_total.php
@@ -96,7 +96,12 @@ class order_total extends base {
           }
         } else {
           // use a template file for output instead of hard-coded HTML
-          require($template->get_template_dir('tpl_modules_order_totals.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_order_totals.php');
+          global $template_dir;
+          if ($template_dir == "yourstore" && defined('CHECKOUT_ONE_ENABLED') && (CHECKOUT_ONE_ENABLED == 'true')) {
+            require(DIR_WS_INCLUDES . '/templates/template_default/templates/tpl_modules_order_totals.php');
+          } else {
+             require($template->get_template_dir('tpl_modules_order_totals.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_order_totals.php');^M
+          }
         }
       }
     }