1. It's the same license, no special license required.
2. Technically no. You won't really find them online (though I'm sure someone will prove me wrong on that). When I was an integrator, the way we (and others) typically got licenses for things we weren't dealers for was to find a friend out of the area who was a dealer. If you know someone out of state for example, who happens to also be an Avigilon dealer, they could hook you up.
3. Third party camera integration varies. Axis is via a direct driver, as are Sony, Pelco, and a couple of others. But I've seen some cameras integrated very well via ONVIF. I remember one Honeywell camera (it's no longer up and running and I don't recall the model) which connected via ONVIF, no direct integration, but had full access to remote zoom/focus, CODEC settings, I/O, audio, etc. Honeywell, of all things.
So in short, integration varies. Honestly the only way to know is to test it.
One question for you: Are you opposed to being an Avigilon dealer? Have you talked to them about becoming a dealer?
We are distributors of Avigilon and so I can tell you that Ethan wrote you correct.
Regarding Q3. Usually cameras with native support have better integration to the Avigilon ACC.
If you have to relay on ONVIF support take note that while most cameras will connect to Avigilon most of them will not have motion detection working as most manufacturers do a poor job in implementing the motion event in ONVIF.
So to make it short you can relay on the Avigilon published list or press the I'm feeling lucky button.
I any case when I have a customer who wants to connect an XYZ camera to Avigilon I always tell him that I can only recommend cameras that I sell and have seen to work well with Avigilon.
All other brands/models that I have not seen, have to be tested before ordering the system.