Answer: Use the Global Thank You page setting in WooCommerce > Settings > Thank You. This will be used for all carts that do not have a Custom Redirect page set.
Answer: The customer is redirected to the custom thank you page without visiting the original thank you page. So only one script would be run.
Answer: The first product in the cart to have a Thank You page will be used for the redirect.
Answer: If multiple products are in the cart and one has a custom thank you page, the customer will be redirected to that page. The Order Details shortcode will still show all products in the order.