I would think that he would need a FO that is within 12" to spot for the Mortar, as the mortar would then be shooting blind and would maybe hit his own troops...
I say this because in reality (I know, I'm bringing the real into this), a mrtar always needs coordinates transmitted to the gunner if the spotter can't see his intended target because of range or conditions (like darkness)...
so, extrapolating this into the game, a FO within 12" would have to spot for the mortar, using whichever penalties are applicable, to get a lock, then transmit the info to the mortar, who then would fire on the target...
the question to me is if the mortar then gets to fire with normal penalties for firing from a FO's data, or if it has to fire with the darkness penalties...
B...
hope that made any sense...