Serbia and Montenegro initially remain under Austro-Hungarian occupation, with internal politics and Budapest's influence leading Vienna to decide against direct annexation. As it attempts to figure out whom to put in power as the head of state for Serbia, a government is established with those who are sympathetic to Vienna, headed by Stojan Ribarac's Narodna stranka and Živojin Perić's Konzervativna stranka, now formed together to be the Narodna konzervativna stranka (with figures such as Vukašin J. Petrović, Vojislav S. Veljković and others joining them) [though this is all assuming different
occupational developments]. As Vienna considers its options, Berlin contacts whom they believe would work best, Prince Mirko Dimitri Petrović-Njegoš, second son of King Nikola I of Montenegro, previously planned to have been proclaimed Crown Prince of Serbia in the event of King Aleksandar Obrenović being without issue, prior to the May Overthrow. Offering him not only Serbia but also Montenegro if he were to collaborate with them, Mirko seems interested, but wishes Vienna's approval, which Berlin ends up obtaining after many long discussions, to the frustration of the Austrians.
Mirko's betrayal is uncovered by the remainder of the Montenegrin government-in-exile and the house of Petrović-Njegoš, fracturing it, with Nikola disowning his son in rage, as Mirko and whatever other sympathetic figures or allies within the government-in-exile departed for the region. Mirko arrives and is declared King Mirko I Petrović-Obrenović of Serbia, not even mentioning Montenegro despite it being part of his territory. Those of the government-in-exile that had joined him, the previously Austrian-established occupational government, anyone that Austria had asked to join the government, and whatever other sympathetic figures existed in the region establish a new government, one of an autocratic nature, suppressing all other political options with any popular support, and if worst came to worst, exiling them too; a dictatorship. Mirko could even get the Jakovljevići and Obren Hristić involved if he really wanted to, if he wanted to have close allies connected to him through his wife, especially since no one from his family would likely join him. Meanwhile, on the Njegoš side, Danilo manages to convince his father, with the help of his mother and sisters, to allow him to renounce his claims and allow Petar Stefan to become heir presumptive, for a multitude of reasons. Zorka and her son Đorđe P. Karađorđević, Milica's son Roman Petrović Romanov (assuming he leaves Crimea to help), Ksenija and Vera all try their damndest in these tough times, while Ana and her husband Franz Joseph of Battenberg would like to try, but are having their own tough times. Thank goodness Jelena was the Queen of Italy.
While initially the CP-supported government manages to keep hold, especially with the help of Berlin and Vienna, Austria eventually has to deal with its own internal developments , and as such, Serbia manages to de facto be independent. Unfortunately, they are completely surrounded on all sides by enemies, ready to pounce on them if they did even the slightest thing wrong. CP Serbia recognizes this, and tries to keep the status quo intact and lower unrest, as to avoid Austria's wrath for the time being, which would probably just delay the inevitable fact that the Serbian people are out for blood. The government tries its best to retain its more absolutist state, in some ways hellbent in keeping itself stable in face of those revolting against the government, and it's a powder keg ready to blow. There may be some attempts to reconcile with those pushing for a conflict, such as conducting some small-scale militarization and perhaps even relenting and allowing more democratic elements back into the government, but they can't keep going like this forever. You'd have folks clamoring for the return of the Karađorđevići (assuming King Peter didn't just die during the war, and honestly, he may just well have), wishing revenge against the state's neighbours, even greater reforms to the political system for greater representation, some even pushing for a republic, those with a more Montenegrin identity wishing for their independence, and more. An unstable, authoritarian and autocratic regime with absolutely no popularity and legitimacy among its citizens, dealing with problems that would overwhelm even the best possible government, a bed for extreme instability - an absolute political mess. Not even use of the church could help Mirko. And that's not even to mention what would happen if Mirko passed away at the same time as he did OTL because of the problems he had with his thyroid gland, leaving his underaged son Mihailo on the throne, along with a regency-turned-diarchal dictatorship that's still struggling to keep the region under check... assuming of course, he and his wife don't go through with their OTL divorce, allowing his eldest living son to join him in Serbia (and, if butterflies allow it, his other two sons).