Hello Rady,
Thank you for interesting our EShop extension. Regarding your questions:
1. Users in EShop is user in joomla with extra information as billing address, shipping address.
2. Joom Credits does not work with EShop. We don't have any integration between these 2 our extensions now.
3. You can only edit an order from back-end side, you can not create new order from back-end side.
If you have any more questions, please don't hesitate to let me know.
Sincerely, Giang