1. You can not. You can stream any audio, video or picture wich are located on your android phone. There are numerous apps for this. But not Spotify, because it is a streaming device itself and has to be included in any of the apps, and they are not allowed. If it is Apple or Spotfiy that are in denial, i dont know.
2. Push your "alt" button on your Mac all along with the volum control upon your screen, Choose the speakers your mac shall delivere to.