You understand me perfectly good
Now this login possibility is useless
Its do nothing, user have to enter data which he provided before during registration one more time
I am not sure if I understand flow proposed by you
I think that user schould not leave confirmation page and later be redirected back
Better solution is to add button (or just link) for login below login and password and when user click it and gave correct login data order form will fill in all data from stored user data and validation works like before, I mean that check out if all requider fields was downloaded into the order form
I hope that its clear know, its diffiult to ecplain by typing
If its still not clear I can prepare some animation to present expected process.