Thanks for writing in! Please understand that Cornerstone is a page builder. When creating a page using Cornerstone, you are not supposed to switched between editors. Doing so would lead you to a corrupted cornerstone data. When you edit a page outside of Cornerstone, any changes you've made will not reflect when you go back and edit the page in Cornerstone. The last saved state in Cornerstone will be displayed and anything that you have added or changed outside of Cornerstone will not display because it is not saved as part of Cornerstone data. It might display on the front end but not in Cornerstone when you edit the page.
If you do not want to use X video element, you may use other player via shortcode added in a content area element or raw content element. So if FV player has a shortcode, then you can use it. You just have to add it while you are editing the page in Cornerstone.
Hope this helps.