1) Yes
2) I've used that order successfully with fuel harvesters before so its probably something on your end. Not sure what though.
3) It's actually quite similar to the previous order, the ship unloads 90% fuel to the colony then moves to a sorium source. The full text is "Unload fuel at colony and move to Sorium Giant" if I remember correctly
4) So assuming you have a ship equipped with sorium harvesters, what I just do is set conditional order: when fuel tanks full, Unload fuel at colony and move to Sorium Giant (the order you described in 3).
5) As far as I know civilian fuel harvesters just turn sorium into money via taxation, the fuel just disappears from the game or something.