Frank, I was thinking last night that there's a better way ... and there is.

Rather than going down the config.core.php replacement, simply place the following define statement just before your script loads application_top.php:
Code:
define ('CHECKOUT_ONE_ENABLED', 'false');
That definition will "preempt" the database setting and will allow your script to run in "normal" checkout mode regardless the configuration of the One-Page Checkout plugin.