Hi,
We will look at that problem tomorrow and if we can reproduce it, we will fix it. I'll reply to this thread once we have more information.
In the mean time, you can change the checkout workflow to
login_address_shipping_payment_coupon_cart_field,confirm_coupon_cart_status_fields,end
for example in the configuration. That will add one more step to the checkout and avoid the problem.
For the registration form, I'm not sure I understand you. It's always displayed on the checkout if the user isn't logged in unless you turn off registrations in joomla.