The checkout post parameter is required to turn the session order into one that’s saved to the database (which is needed for the account and subsequent steps).
If I simply copy/paste this form with the set_guest_checkout = 1 or checkout = 1 param and “shop/purchase/checkout/?step=address&xpck_lang=en” as action it doesn’t work on the “cart” step, there is a 302 to shop/purchase/checkout/?step=cart
Correct. Only the account step handles set_guest_checkout, and because you haven’t yet POSTed checkout=1 to the cart you do not yet have access to the account step, so it redirects back to the cart without performing any actions.
(In that case, if a user is logged in before they arrive in the checkout, that still registers the order to their account; they just don’t have the option to login in the checkout itself.)