Unfortunately, we are not aware why that 3rd party plugin is not working with Cornerstone. It is nearly impossible to give support for 3rd party plugins as we do not know the ins and outs of the plugin code. You will need to contact the developer of the plugin to follow up.
Meanwhile, I suggest that you take a look at the Modern Events Calendar extension which is bundled with our theme and you can use it free of charge.
Kindly open up new threads for additional questions as it will help us to focus on each issue and give you a better support which you deserve. Having a long threads makes the maintaining job harder and also it will be harder for the other customers to find the correct information if they have similar issues. You are always welcomed to reply to this thread to follow up the same question.