Hi Son,
First of all please make sure that you are using the same version 6.1.5 and Cornerstone version 3.1.5. If not please update.
I am not sure about the extra space it might be a problem caused by the custom CSS or JavaScript code or maybe a child theme. Please remove any possible CSS or JavaScript code that you might have added to the website and also change the website theme from the child X theme to The Parent one.
To make a section fullwidth, you need to go to the layout tab of the Cornerstone, expand the section in question and click on the row settings:
There you will find options to remove the inline container and extra margins:
kindly get back to us with the result of the steps above and URL/User/Pass of your WordPress dashboard using the Secure Note functionality of the post to follow up the case if you still have problems.
Thank you.