Great questions, here are some of my thoughts:
Finding users with the necessary hardware to test: I wonder how responsive the reddit community would be here? https://www.reddit.com/r/daydream/
Observing usability tests: Google’s Daydream 2.0 updates can hopefully help by allowing you to easily cast your experience allowing for better observations.
Remote testing: As WebVR becomes more accessible, maybe that could become a solution to help with remote testing via mobile phone VR headsets (assuming your testers have their own headsets). Or if you’re able to ship an experimental APK file for Daydream/Android testers to download and use that could be one way for them to access your prototype designs.
Keep up the great work!