When you click on Check Out and Allow guest checkout is disabled you are directed to the Joomla Login screen and You have to create a new user account which is very clumsy
when Allow guest checkout is enabled the obvious route is check out as a guest unless you click on log in and then choose create an account which does not make sense.
The creation of a new joomla user account needs to be intergrated better eg. a check box labelled "create an account" which enables the password and verify password boxes with a check for email exists as a user account
As a suggestion - The top of the check out screen needs to show 3 simple options
Create an account / I have an account log in / Check out as guest (if enabled)
that takes you to the correct Joomla login or registration screen
index.php/component/users/login
index.php/component/users/registration