Pulling in the customer (and presumably address-book) records from the live store that didn't have OPC installed has, though, wiped out the guest-related records. You can cause OPC's admin initialization script to recreate these records by running the following query (either in phpMyAdmin, where you need also to include the site's DB_PREFIX, or via the admin's Install SQL Patches):
Code:
DELETE FROM configuration WHERE configuration_key IN ('CHECKOUT_ONE_GUEST_CUSTOMER_ID', 'CHECKOUT_ONE_GUEST_BILLTO_ADDRESS_BOOK_ID', 'CHECKOUT_ONE_GUEST_SENDTO_ADDRESS_BOOK_ID');
I don't know what's causing the checkout/confirmation loop. Is there anything in the browser's console log, seen by pressing F12 on most browsers then inspecting the 'Console' tab's information.