Summer grasses / All that remains / Of soldiers' dreams. - Basho.
P.6 says "You can play a Location card even if you are not in the front-most Location card."
P. 15 says "To play a new Location card, there must be a Soldier in the front-most Location."
Seems to contradict. I'm guessing that in the first statement the "you" is referring to Player Soldiers only and the second statement is referring to any of the three Soldier types.
Therefore the rule is that there must be at least one Soldier of any type in the front-most Location to play the next Location card.
Is that it?
You got it - a soldier (any soldier - NPS, player, Squad) must be in the front-most location. The first "You" in the P. 6 case is actually referring to "you" as a player controlling a soldier (any kind).
My guess is that this is mostly a game balance feature to prevent players from playing cards with their soldiers well away from the "front line", so that the spawned-in enemy hostiles would be forced to move towards the player-controlled/secured location. If this was permitted the players could make sure was a 0 reinforce location, or one of the various locations which are advantageous for players (such as High Ground), or otherwise be well set up with sniper and ranged weapons to hit the advancing enemy as they moved up.