One can certainly delay German unification. For how long is a difficult question, though - we after all have no comparative data from real histroy to base our estimates on
But with the NGC and the Zollverein firmly in place, I dont think the three south german states (four if you count the southern parts of Hesse-Darmstadt) can hold out as independent states much longer. Hell, Baden already did apply to join the NCG in 1869 and got rejected, and I think it would be ready to do so later on, too. It didnt get all that well along with the other two southern states, anyway. And Baden and Württemberg? Prussia has the whip hand in the Zollverein, and could certainly use it against just two further states.
Of course - only if Prussia wants to do so, and I think thats the problem. In such a scenario, with no accidental war with France (and I see it as such), Bismarck may well be the main hinderance for further German unification. Him, and maybe King William I of Prussia. Both after all didnt want an united Germany, but a Greater Prussia - and they had achieved that, the NGC was just that. But Williams heir, Frederick III, was somewhat associated with the national liberals (far less then it is often made out to be, but that isnt all that important now), and in any case greatly dislikes Bismarck.
So if William dies as per OTL 1888, then Frederick III, or if he also dies in the same year as per OTL William II might certainly set in motion events to lead to the unification of Germany. Of course, it might also happen earlier: With the NGC being nearly all of Germany-outside-Austria already, and the Zollverein in place, people might begin to question the sense of the setup. The topic might certainly come up in the Zollverein conferences. So it might happen in the 70s already regardless.
And there wont be a Prussian-Austrian showdown. That already happened in 1866, the matter is settled, and sealed with the Austrian-Hungarian Ausgleich. Austria has form that point onwards anymore no place in Germany anymore, save for the possibility of the breakdown of the Double Monarchy. im unsure about Russia. An 80s without united Germany will be a different situation, diplomatically, after all. OTOH, France might certainly go to war. IBC argues eloquently that Napoleon III wasnt all that much for war, but he wasnt in control anymore (hed still be Emperor, but in the Liberal Empire not as political dictator anymore). So whatever government is in place in France will most certainly start a war, as they did IOTL. The big question is - will the Liberal Empire governments have used the time to reform and modernise their troops or not? If yes, then a French victroy is the most likely outcome, if not then a German victroy as IOTL is pretty much assured.