I am having the same problem.
The main menu when it converts to the mobile icon will not drop down or open the menu on mobile platforms.
I tried your recommendation. It didn’t work. I saved the cornestone.php file before I deleted it.
I went through my FTP and deleted the directory.
I then visited my home page and saw that the mobile icon has nothing to do with Cornerstone plugin but is related to the X Theme.
I re-uploaded the file cornestone.php file and do not know what to do.
UPDATE: It is the X Theme Javascript code for the mobile icon that is not working because when I uninstalled the X Theme and installed the 2017 WP Theme the mobile icon worked well. It has nothing to do with a conflict due to Cornerstone being active.
GOOD NEWS!!!
I just saw a new UPDATE for X THEME and as soon as I updated it, the issue has been resolved!!!
THANK YOU!!!