1.Yes, all updates are only related to the hardware - The ASIAIR device. But some features such as Video Mode, ASI6200 camera support are only available for the ASIAIR PRO.
2.Yes, if you bought from another user, the device is totally yours now.
3.Nope, the license is bound to the device, it cannot be transferred from gen 1(ASIAIR) to gen 2(ASIAIR PRO).
4.Users of the ASIAIR can make an instance of buying or selling their ASIAIR to others. There is no limitation from ZWO.
5.With the correct ASIAIR OS was installed on the SD card, yes, it will work.
6.Since the ASIAIR PRO was released we do not provide the OS/Software Only solution anymore.