Overriding default template from customization section .

I am trying to override my default templates from customization section, I am using code to do that, but if I am using it I am unable to assign a template to the edit-page page, Can anyone give an idea how both the customization section and edit-page assign template work. I want to set the template when I am creating a page and after assigning it I want to override. Consider I have a blog page, I want to assign it archive.php template and ten want to override It from customization section. There is the particular condition where I want it to work.

link to code