One way that you could do this would be to move your set session value server behaviors on your confirm page to be above the validation code. You may need to add a few in also in case there are values you are currently not setting in session variables.
If the values from the form are set in session variables before they are validated you can just set the value of the fields on your checkout page to be these session variables. That way if these variables are set they will be the default values on the checkout page.