Hi Richard,
I will explain a bit here:
1 - To have ?tp=1 works and you're able to see positions on frontend, you need to enable the option Preview Module Positions in template options:
2 - To see the preview of module positions in backend, we do support this option in Layout settings of template:
3 - About the sub-layout: suppose that you have a category blog page and this blog page will have layout of template style you're assigning to, when you click to open an article from this blog page, it will inherit the same layout.
Now, what if you want to have specific layout for article detailed page? you can't achieve this in default menu assignment in Joomla. In this case, the 'sub-layout' will help. When you select a sub-layout, the article page will use this sub-layout instead of the layout assigned to blog page.
I hope these make senses.