This thread is super confusing, before we continue with this path, let's clarify your goals and find out what it is the easiest way for you to achieve which will end with a good result.
1 - If you want to create a page without header or footer, you can select the template blank no container no header no footer.
2- You can edit the page with Cornerstone, add a section, add a background to the section. Click on the customize tab of the section > Inline CSS and add : min-height: 100vh;
3- You can than edit your page in Cornerstone and divide your row using custom grid and add there 1/5+3/5+1/5
This will give you a nice landing page with NO SIDEBAR, If you want a side bar, based on your site I would suggest to also NOT USE a sidebar, but build a sidebar looking column on Cornerstone.
Hope this helps, if it does not, please provide an example of a page similar to what you want.