Hello,
1) That field is the custom metabox field, you may check in xstore/framework/custom-metaboxes.php file. If you want to show widget area at the bottom for every category you can implement it by additional customization if you have appropriate skills.
2) Sorry but we it’s not depends on the theme, that cache is browser cache.
3) You can use the following plugin to implement this https://wordpress.org/plugins/woocommerce-products-filter/
Regards