Hi,
1. You can add the HikaShop menu "user control panel", in this panel there is a link to the user orders.
If you want a direct link, just get the url to the orders and create a new menu/button as "external url" and paste the url.
2. Same thing than previously, create a button in the desired view, and as url use the last url of the checkout workflow.
3. Could you provide the full message ?
To translate you can, search that string in the english file for example, then copy the translation key, and in your language file add the translation key, and its translation in the override part of the language manager popup.