eBay has their own definition of refurbished an it it not what one considers as really refurbished or made like new. For eBay "refurbished" means it looks like new, has several cycles, and has an AllState (not DJI) warranty. So it is USED, but...
Yes, you can ... no problem at all.
Yes, you need the app to fly the drone.
The app is pre-loaded on the controller and it does not require internet connection to fly, but you will need internet connection for setup before flying the first time...
Start with the manual for the drone. You have a lot going on in your post, but it is going to come down to what drone model you are receiving, and you learning about it. This is best done by reading the manual. Then if you have questions or...
in the end the reason I cancelled my order was DJI telling me they are not prepared to repair them in the U.S. and I would have to send it back to Korea for repair because they do not have the parts and ability to fix it in the U.S. A few weeks...