[Pfor]

Rather than mess with the 'indirect fire' mechanism, I would make the 'direct fire' option more attractive. From what I have read, the most usual FO for these mortars was the mortar's own squad leader, from a vantage point within 100m, using hand signals.

What if the light mortar had a Line of Sight calculated from the hex immediately in front of it?

This could allow the mortar itself to stay in cover yet still 'see' its target. Obviously, this hex would be fixed for the entire move, based on its direction at the start of the turn (i.e. no such benefit if rotated), since there weren't enough spare crewmen to place one in each of the adjacent six hexes. For simplicity, the spotter of course would never ever be spotted!