Yes, getting the head size is insufficient, and will printing an A4 head bending to the face is a possible attack.
The eyes closer than nose method I didn't continue as it is possible to detect a face that is slightly tilted to the camera (e.g. left eye/spectacle is closer than nose to the camera). Maybe I should treat that as a fake or hint person to face camera.
The binning one I probably will try as not sure what to do with the depth so far.
I guess there will be some dirty coding (mixing methods) to achieve what I want. Thanks,