Hi,
1.
If the order has not yet been created, next time the user comes and log on the website, he will have his cart and address already there for him to checkout.
If the order has been created, if will be able to see it in his orders listing. You can create a menu item of the type user control panel via the joomla menu manager for that. You can turn on the option "Payment of orders not paid immediately" in the configuration if you want the user to be able to pay orders not paid yet.
2.
You can create a menu item of the type user control panel via the joomla menu manager for that. You could create a custom field of the table "order" via the menu Display->Custom fields so that you could edit it via the order of the user.
Then, in the order details page, you could display it with a simple line of code in the file "show" of the view order" of the front end via the menu Display->Views:
<?php echo $this->element->COLMUN_NAME; ?>
where COLMUN_NAME would be the column name of the custom field.