Hi
1) Go to WooCommerce > Settings > Products > Display and edit your single product image size, then use the regenerate thumbnails plugin
2) If you’ve set images on the variations themselves, then the image will change to that when you select a variation
3) Yes, because the main image changes to the one you have set on the variation, but you also have that image in your product gallery. You should only use one or the other, I’d suggest removing images from the product gallery and only using the variation image
4) Again, change the woocommerce image sizes then regenerate thumbnails
5) That’s because it can not be added via AJAX, as there is additional information that needs to be sent to the server about which variation has been chosen, and therefore the page needs to reload
– Kyle