set featured video from frontend

Um I know nothing about your implementation. So lets assume you just want an <input type="text"> on your frontend to pass an video url to the plugin and save it as featured video. Remember that you still need an featured image to be specified.

I think the simplest way would be to pass your url to the plugin using jQuery & AJAX. WordPress handles all ajax requests through admin-ajax.php in the backend, even if the request is sent from the frontend. This way you do not need to include any of the plugin files on the frontend.

$.post( ajaxurl, {
	'action'    : 'fvp_ajax',
	'id'        : $('#post_ID').val(),
	'fvp_nonce' : $('#fvp_nonce').val(),
	'fvp_video' : $('#fvp_video').val(),
	'fvp_sec'   : $('#fvp_sec').val()
});

ajaxurl needs to be specified before, it is just available in the backend by default. Use admin_url('admin-ajax.php'). fvp_nonce needs to be generated by wp_create_nonce('fvp_nonce'). You can use wp_nonce_field for it. id is the posts id you want the video to add to, fvp_video is the url, fvp_sec the (optional) fallback video url.

I have no idea how skilled you are with WordPress, this is just a rough mockup of the way I would approach it. This wasn’t tested! Still, hope I could help,
Alex