And is it possible to distribute a GVA Spotify stream to other NL speakers?
Sadly no, and I don't expect this to change.
When you start a Spotify stream through GVA it will always start the stream as a Chromecast stream. It is not possible to use Multiroom with a Chromecast stream.
For this to change we need either Google to allow GVA to start a Spotify Connect stream (Never going to happen, as this is a direct competitor to their own Chromecast), or Google should allow Chromecast to be used in Multiroom. The latter could be the way forward, but there might be technical limitations at play here. Chromecast can not extend to more speakers without using a pre-defined group of speakers at the moment.
So basically what you are seeking is a mix of the Chromecast and Multiroom functionality. For this to happen Google needs to change things, and B&O needs to implement it. Not something I see happening anytime soon, primarily because B&O doesn't have the power to make Google change.