Hello,
1) For this you would have to modify the file ~CMSModules\Ecommerce\Controls\ShoppingCart\
ShoppingCartCheckRegistration.ascx. This is where this step is generated.
2) To achieve your desired result, you will need to do some custom development. You will need to create a custom checkout step and then combine all of the default checkout step controls into one step. All of the code files you will need can be found in the following location: " ~\CMSModules\Ecommerce\Controls\ShoppingCart\". I'm afraid there is not a whole lot of documentation for this approach, but this
documentation should get you started in the right direction.
Best regards,
Boris Pocatko