Detecting user camera direction in volume?

Does scene camera accurately reflect the user’s position or gaze direction in a volume?

Unfortunately, no. Apps do not have access to ARKit features (among which is user head pose) without an ImmersiveSpace open, which means an Unbounded VolumeCamera in Unity. The gaze ray is also not available on SpatialPointerEvent input for bounded volumes, either. Head pose and gaze ray both require an open ImmersiveSpace.