Remove alternate checkout completion page

Ubercart has the option to supply an "Alternate checkout completion page" via admin/store/settings/checkout.

The standard uc_cart_checkout_complete() runs prior to the redirect to this custom completion page. This does, among other things, the following:

unset($_SESSION['uc_checkout'][$order->order_id], $_SESSION['cart_order']);

How is a store meant to reliably/cleanly fetch order information necessary to display relevant information at their custom completion page, when the relevant session data (in particular $_SESSION['cart_order']) is seemingly nuked before the user lands there? (Bare in mind that one can for an example allow anonymous checkout without logging the user in after checkout - so one will not necessarily have a userid to query for their "last order" or similar hacks either.)

Last time I looked into it... the only solution I found that ensured consistent behaviour, in the time I had at my disposal, was to leave the checkout completion page path at default and use hook_menu_alter() to replace the callback with a custom implementation. That's obviously very hacky and less than ideal in terms of upgradability and such though.

From what I can see the cart related functionality uses $_SESSION['uc_cart_id'] to load the cart contents, and there is a safeguard in place for the checkout page where it will unset $_SESSION['cart_order'] and throw the user back to their cart if the order has changed status.

So if I were to supply a patch to allow some way of passing order info on to the "Alternate checkout completion page" - would it be acceptable to let $_SESSION['cart_order'] live beyond uc_cart_checkout_complete() and leave it up to the custom completion page to unset it? Or should we introduce a new session variable for this purpose alone - such as $_SESSION['last_completed_order']?

The latter would perhaps be the cleanest - to avoid users getting a "Your session has expired or is no longer valid. Please review your order and try again." if they try to shop again on the same session, and the custom completion page failed to unset $_SESSION['cart_order'] (the developer forgot to add it, or whatever...).