> Instruct the browser to re-load the page upon navigating back (cacheability headers), identify the order using an ID in the URL, then when reloading detect its already-submitted state on the server
Re-loading the page on navigating back would be done using cacheability headers. This is the most shaky part, and I'm not sure if it is possible today. If this does indeed not work, then this would be one of the "things that Javascript has solved that the non-JS web is still stuck with" I meantioned in my other post, i.e. one of the reasons that JS is more popular than non-JS pages today.
Identifying the order using an ID in the URL is standard practice everywhere.
When the order page gets requested, the server would take that ID, look the order up in the database and see that it is already submitted, responding with a page that has its submit button disabled.
The must have js part for me starts where one can open the store in multiple tabs then add and remove things on the first two and check out on the 3rd tab.
And how would one do that without using JS?