What you are suggesting is just not possible..
The call $op->page_content()
gives all content for all pages into a one-pager template.
If the class would generate separate sections you would get a full website section(2) output in that call.
Doing it the way you are suggesting you should use something like globalBlocks
for all blocks/sections in your template and create the menu hardcoded.
You are making a mistake in thinking you need a content section for the full screen images.
In this template these images are backgrounds, set by CSS, not set as content.
The output of the onepager class does generate a perfect output to do exactly what you want.
You just need to remove the .wrap and .container from the template/index.php and understand the CSS included in your template and modify it with the correct id and class selectors.