Maybe naruto controlling kyubii is simpler than we thought?
The demon fox doesnt appear to be too happy that madara controlled him to do his bidding, and im sure that is why naruto has trouble controlling him too, because he simply wants to get out and do his own thing, however were missing a rather obvious point as the reader we know all the information that most characters don't
For example only a few knew madara controlled the 9tails, and most are dead or not allies.
Naruto certainly doesnt know tobi is madara.
Im sure the 9tails will follow what naruto says if naruto find out who tobi is and says "as long as i control you, and you dont overtake my body, i will allow you to use your power through me to kill madara"
even better is this ties in nicely with itachis gift maybe.
Itachis gift could simply be a way (as ive stated before) that madara cannot control the kyubii, and naruto as a medium keeps the fox safe from being controlled (therefore naruto) and the fox's power symbiotically keeps naruto safe with its amazing power.
The kyubii has been seen as a horrible entity in naruto, but in reality it theoretically can work as a perfect symbiotic system.
Im also sure the key was given to open up the kyubiis full potential once this pact and agreement was made.