Presuming you are planning on using your Astrobox in conjunction with the AstroPrint Cloud Platform AND that you are NOT interested in operating the Astrobox directly/locally via the hotspot, the short answer is NO, you don't necessarily NEED the extra dongle for the local hotspot.
Indeed. That should work.
Static would be best.
Correct. Once the astrobox is setup, and you pair it with the local WiFi network and your cloud account, you should be good to go!
Static would probably be the safest, but it shouldn't be a problem. It's been a long time since I setup an Astrobox in this manner, so I am not 100% sure about this one.
In either case, let us know how it turns out
Welcome to the club!