1- In the new versions of the Cornerstone, due to complexities of the 3rd party plugins view for the content builder, we will not be able to show the preview and you need to check the case in the front end of the website. It is the case also for plugins such as the Essential Grid or The Grid. Here is the link of the news which we announced the matter:
2- Please make sure that you have the latest version of the Revolution Slider and you add the video using this method.
If you still have problems please add a new Revolution Slider, add the video there and add a new page with the slider in it. then get back to us with the name of the page and your website URL to follow up the case.