Hi Ousai,
1 and 3 0.000 0.000 1.000 is here the normal vector on the sensor, which is defined to be in the positive z- direction. This does not directly define the orientation of the sensor. The orientation is indeed by the vector [-0,35615; 0,90862; -0,21814] for example. Which is also a unity vector in 3-d. The sensor can then be rotated to have it's normal vector match that orientation.
2. You can find a short discussion on OPM in the following thread : FMEG FORWARD Modeling
4. I don't know this, someone else might chime in.
Kind regards,
Steven