[Brightcolours]

That under-OVF sensor may just be a sensor to switch the LCD off when you bring the OVF to your eye.

Most definitely not. Imagine the camera in LiveView and you a proud Penatxian comes too close to the sensor with his hand or whatever: LV stops and needs to be started again. So far my guess is a hybrid EVF.

You are overthinking it. It is not for use during live view.
Look at the back of the EOS 450D I used to have:

See that sensor under the OVF? Looks kinda the same, doesn't it? During OVF use, the LCD is on when you don't put it to your eye, and off when you do put the OVF to your eye. That is the only thing that sensor does. It cuts out the distracting light from the LCD when you look through the OVF, without having to do anything else.
Of course it will not cut out liveview when in liveview, why would they use that sensor in such an obstructive way?