Hi and thanks for the welcome!

Here are your answers (very interesting questions indeed)

1. Yes, the whole system will be controllable from APIs. At launch we'll have just socket.io APIs, and in few months I plan to mirror them also with REST ones. They are already documented, and should give you a better idea on the whole architecture:
https://github.com/volumio/Volumio2/wiki/WebSockets-API-Reference

2. The UI is fully powered by Angular.Js and gets sends data via WebSockets (socket.io).

3. I don't think there will be WOL, as this is something that requires a specific hardware state. Hardware wise we now have ON\OFF.
But this could be an interesting feature to add in the future (but sure it will require hardware redesign)...

4. Absolutely, you have full control on volume, and you can retrieve it (together with all playback related info) from the getState API call

5. Yes, with Socket.io

6. Yes indeed. Every AxiomAir device uses the same APIs to talk with the others, and so can you.

Hope I replied with clarity to your question. Really eager to know your feedback on the architecture we set up.