Commerce Allows User to Edit Cart After Completion of Checkout
Hmmm, this is definitely something that deserves to be fixed in Commerce itself. I'm going to open an issue there and outline a strategy that essentially adds a form alter hook to the Cart module to redirect back to /cart in the event that the shopping cart form is being built / submitted for an order that is in an unsupported status. I can't say exactly how I'd do it yet, but I suppose it would involve iterating through the checkout pages to allow shopping cart form submissions for any order that hasn't yet passed a checkout page that doesn't have back buttons (e.g. the Payment and Completed pages).