As long as a payment has not yet been completed, a field with the checkoutUrl is displayed in the administration backend for the order.

Is it possible to access this URL in the shopware frontend and make it available as a link to the open order in the customer account page, for example?

If, for example, the customer does not complete a card payment immediately for whatever reason, he cannot initiate the process again or complete it subsequently. As long as the transaction is not aborted/cancelled by Mollie after a certain time anyway, the customer would technically have the option to complete the payment via the checkoutUrl link... but not if it can only be found in the admin backend.