A.1) When you are doing a p2p video call to a person, the only traffic that will be used server-side is for logging start and end time on the server (and the like). The video traffic won't affect the 10Gig.
A.2) You should be creating one account for your app - so: both.
A.3) Video Conference with more than 2 participants is not supported natively in QuickBlox, but you can bind your video signal to more than one instance, allowing you to stream your video to multiple participants at the same time. Of course, with higher user count, the necessary bandwidth of every user increases. If you have p2p between six users, you have to push five incoming and five outgoing video signals through each participant's internet connection. (You should think of a clever way to downsize the signal on the sender side.)