If that is the only product that has product_info immediately after main_page=, then for now concentrate on monitoring that product related to changes proposed. If other product have something different, then whatever works for product_info will in a way work for the other(s) (supposedly. :))
It may end up being a little difficult to tell what exactly is needed because of the mixed state of the software.
Could you please post the contents of includes/modules/pages/checkout_shipping/header_php.php? Same way as above/before?
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