Hello,
If you "Default registration view" dropdown is empty, then you have to define your checkout access mode, see my screenshot :
This setup will allow you to define authorized way to access your checkout, and on the frontend you must have a selector to allow your customer to choose among possible access, see the frontend screenshot :
So, from this different form can't disappear, if you have define all these different settings, then by using the selector button your user will be able to choose his way to your checkout, and the called form will be displayed.
Hope that this details will allow you to progress on this subject.
Regards