Yes, you can, but only one rule per product applies. The rule is chosen following a specific priority order. Read more about rule priority order on this page.
No, the shipping cost is a fixed amount and it is applied to the product only once. It’s the product cost (optional) in each rule which is multiplied by the product quantity.
Yes, the plugin works well. It does not interfere with pages built with Elementor, so you can use both plugins on your site without any issues.