how well does the RTH work when suddenly lost connection in heavy fog/cloud?Would it still RTH if the visual sensor thinks it is surounded by obstacles?
Is it better to turn off the Obstacle sensing, and set RTH altitude to 0M for moutain top flying?

It is not recommended to fly in fog because the GPS signal will be weakened and the sensing system will not work well in this weather condition.

The RTH will be unaffected by fog or cloud, it will still come home, but beware the clouds may have rocks in them.