Would it be correct to say that units in adjacent hexes always have line of sight to each other, regardless of terrain (10.6.1)?
I don't think that matters. You cannot make a ranged attack against adjacent enemy unit, except as adjacent defensive support against enemy units in a frozen lake hex. [10.3.6]
Adjacent hexes are always spotted. [10.6.1].
Anti tank fire can be performed against an adjacent armoured unit, or against an assault which includes armoured units. [11.0]
Am I missing something that requires LOS against adjacent hex?