I want to use a product thumbnail completely different than the product image

There’s several ways you can do it. My suggestion would be to create a custom meta box that either accepts a either the image url or you pass the id of the image you want to use. Then within the template you can use the wp_get_attachement_image($args) function. https://developer.wordpress.org/plugins/metadata/custom-meta-boxes/ https://developer.wordpress.org/reference/functions/wp_get_attachment_image/

Add custom attribute to product’s HTML to woocommerce cart page

You want to override the file, not edit it directly. How to Edit Files Edit files in an upgrade-safe way using overrides. Copy the template into a directory within your theme named /woocommerce keeping the same file structure but removing the /templates/ subdirectory. Example: To override the cart.php file… copy: wp-content/plugins/woocommerce/templates/cart/cart.php to wp-content/themes/yourtheme/woocommerce/cart/cart.php The copied … Read more