1) All officers can turn out equal, except for their specific chi ability, which some share any way. When I played, their were alot of zhou yun's and cao pi's. Cao pi is nice, because his sword can combo with chain. A continual loop of wide spread, long range slashes can tear through most of the game. Pair it up with control++ (I think there is a ++ if not its +) and fury armor and you have one of the most op combos in the game.
Personally, I like Lu xun more since he can use the same sword well and his actual sword works well in the air to compliment the ground based power house of the longsword. Also Sun ce with aero+ is also a great combo, able to kill officers in one combo.
2) You should always bring allied officers. They arent very helpful, but they gain lvls so you dont have to gain them yourself. The kill part I will discuss in the next question, since it ties in with it.
3) Deeds are your exp for the level. If I remember, the max you can get is 2195 per mission ( might be more but it will make more sense).
When you kill soldiers and officers you get deeds. Whne you max out your deeds for the mission while you are still in it, then the deeds get split between your allied officers (told you it ties in here).
When your officers kill soldiers or other officers, it adds to your deed total, not their own.
I think its around 1000 kills for max deeds for yourself, but its been a while so I could be off alot number wise =/
Hope this helps answer your questions, feel free to let me know if im wrong any where!