There is no difference in designs in Design Extender and Site Sculptor.
Design Extender does use the latest version of PowerCMS with site sculptor to match PowerCMS builder, so it avoids any upgrade steps you are going through now.
You should be using admin_cms after the upgrade and can delete the admin folder.
A blank page means an error on the page... perhaps some files weren't uploaded. Make sure you have uploaded all of the files and the admin_cms section should work. If not we can open a support incident and have someone look into it with you.