Hello,
1) It depends on your wish. If you upload images to variation gallery they appear only if you choose the variation. If you want to show these images in product gallery even if client does not select the variation then you need to upload them also to the product gallery
2) It displays if the user does not choose the variation
3) I passed the issue to our developers.
4) Contact plugin author that you use to add that functionality (I suppose Facebook for WooCommerce plugin author) because it’s not theme built-in option.
Regards