Is it possible to add google TV/ Android TV to nebula?
Not really. Nebula isn’t a separate OS, it’s “just” an app that manages the glasses, and in certain cases (MR mode, AirCast Side Screen mode) it takes control of the secondary display, either by directly drawing to it (AirCast Side Screen mode - it records the phone display, then draws that into a small segment of the secondary display, i.e. the glasses), or by passing a reference of it to the Unity engine (MR mode).
As such, Nebula can’t run Google TV/Android TV, since it’s a completely separate OS with its own services and such. The closest you could get is by using an Android TV dongle, with a GoFanco HDMI adapter - but it wouldn’t be too useful on the go, as Android TV does not support Miracast or AirPlay, only Chromecast, and Chromecast requires a network connection (among other things) to work.
No, it is not possible. Please notice that 2D Android apps cannot be opened in Nebula’s MR space.