The code you have added has an undefined constant which might create an issue with the latest PHP versions. Instead, you may use the code given below which will allow you to add media to the short description and description.
add_filter('dokan_product_short_description', 'add_media_button');
add_filter('dokan_product_description', 'add_media_button');
function add_media_button(){
return array(
'media_buttons' => true
);
}
After adding these codes, you shall see the add media button on your vendor dashboard product short and long description fields.
Now, to remove the file type and size upload problem, you may use a third-party plugin rather than using a hard code. You can also restrict file type and size upload by user roles. I would suggest using WP Upload Restriction plugin. With this plugin, you can limit upload size and type. Though this is not tested by me since it’s a third-party plugin but it should work.
To change the default media player style, you may take the help of this article: https://www.cssigniter.com/css-style-guide-for-the-default-wordpress-media-player/
Hope this helps.