Hi,
About the S6:
The Android Chrome browser is blacklisting the GPU of the Samsung S6 and therefore no WebGL is available.
You can change that by following these instructions:
Web VR on Note 4
About the iPhone 5:
iOS 8 or higher is required, WebGL support with introduced with iOS 8. I assume your customer is using an older version.
About forcing VR on startup:
Make a kind of intro screen or intro button and call the enterVR() action when clicking on it.
Best regards,
Klaus