The issue is that the login form fields, which are in the user/registration view file are inside the checkout form.
So if you want to include the seblod form, it means that you need to remove the whole checkout form but you need it for the rest of the checkout elements.
In that case, I think that it will be way easier to simply redirect the user to the seblod registration page if the user reachs the checkout and is not yet logged in. You can add such redirect in the "login" file of the view "checkout" via the menu Display>Views.