I have a noob question about the diplomat ability.
So if you have a figure on your side with the Diplomat ability, no one on that squad can be attacked by a range-weapon until the Diplomat is taken out?
Almost exactly the opposite. You cannot attack the diplomat if you have line of sight to any other enemies. But you can attack anyone besides the diplomat with a ranged weapon, pretending the diplomat is not even there (since the diplomat does not count as the nearest enemy). The diplomat would still provide cover for anyone behind him, though, and normal targeting rules apply to the rest of the squad.
Diplomats are especially useful for 2 reasons:
First, they are good at scoring 'gambit' points in competitive play. If you aren't playing competitively (DCI), don't worry about it.
Second, if they are adjacent to an enemy, that enemy cannot attack anyone else (since you always have to attack adjacent enemies first). And the enemy cannot attack the diplomat if they have line of sight to someone else. So the diplomat can force an enemy to either move or sit there and be useless. For someone like the Emperor on the Throne (who cannot move), or Savage characters (who have to move adjacent to enemies if they can), diplomats are especially nasty.