One last avenue of diplomacy and investigation lay open. The lone legal adult amongst the armed assassins was Mehmed Mehmedbašić.
Following the assassination, Mehmedbašić fled to Montenegro where he was arrested by the police. In Montenegrin custody, Mehmedbašić confessed to a wider conspiracy including an irredentist Serb terrorist planning meeting in Toulouse, France. Learning of the arrest but not of the confession, Austria-Hungary asked Montenegro to honor their mutual extradition treaty and hand over the assassin. After Montenegro shared Mehmedbašić’s confession with the French Ambassador, the problem of extradition was solved by permitting Mehmedbašić to “escape” to Serbia.
What would have happened if Austian spies had discovered this information? If it had been publicly known that a wider conspiracy aided by serbia could have been proven.
In OTL, Austria couldn't formally prove these charges, and though the Tzar and his foreign minister told Serbia to accept the ultimatum sent by Austria, public opinion would force the Austrians to soften their stance.
What if, armed with this information, Austria again presents its ultimatum, and now has the court of public opinion firmly on its side. Will Russia still wash its hands of Serbia to a point? Will it pressure Serbia to accept? What about France? Can they risk a war if it's own people know that they would be fighting to aid a government abetting assassins?
I'm curious if Austria could have demnded harsher terms in this instance, or simply had enough support, even from Entente powers to force Serbian acceptance and thus avoid WWI, at least in 1914.