1) I added the following custom css in your Theme Options -> Custom CSS This will set the full width of the full-width template to a maximum of 1300px.
2) You need to use the Theme4press shortcode manager button found at content editor. The button with a number 4.
See screenshot. http://prntscr.com/7dq395
Click on it and it will popup with a menu for you to select the shortcode that you need.
Select the content boxes shortcode. You will see that you are able to select 3 boxes in a row.
Do not edit demo pages, leave them as an example.
I created a new page called test, with two rows of content boxes.
3) Sorry, I do not know what you are referring to when you mentioned “the size of heading on feature items.”
Please try out the shortcodes manager, and templates..etc, get familiarize with how the theme works.
You need to be familiarize with the theme in order to design your own website. Theme Options are impossible to cover everything. Most of the elements are created by shortcodes.
Every theme is a tool and you need to learn how to use it.
We cannot be providing assistance in designing your website, we can only explain how to use the theme.