In October 1912 after the Bulgarians have decisively defeated the Ottomans at the Battle of Kirk Kilisse, Austria-Hungary decides that the Balkan League has gone too far, and demands that they agree to a ceasefire with the Ottoman Empire, and that Serbia withdraw from Albania and Bulgaria withdraw from Eastern Thrace. Austrian diplomats don't clarify what they mean by "Albania", which infuriates the Serbs even more because they think Austria is trying to force them to withdraw from Kosovo. However, it also informs the Greeks that the Dual Monarchy is not opposed to Greek control of North Epirus (under the reasoning that it's better to have the Greeks control it than the Serbs or Italians). To show how serious the demand is, Austria-Hungary begins to mobilize its army.
Russia decides that this is the moment to take a stand and make up for the humiliation of 1905. After some hard negotiations and arm-twisting, Serbia, Bulgaria, and Russia come to an agreement. Serbia and Bulgaria will continue their war on the Ottoman Empire, Serbia will get its promised port in Albania and Macedonia will be split by the two countries according to the pre-war agreement. Bulgaria would get Western Thrace and part of Eastern Thrace but most of Eastern Thrace along with Constantinople would go to Russia if the Ottomans fall. Russia will declare war on Austria-Hungary if Austria-Hungary goes to war against Serbia and Bulgaria. If Austria-Hungary is defeated Serbia will also get Bosnia.
Meanwhile, Germany agrees to defend Austria from Russia and starts mobilizing too. Both the Germans and Austrians inform the Ottomans that they aren't interested in preserving Ottoman control of Albania and demand that the Ottomans agree to a ceasefire with Greece, which happens. Austria-Hungary follows through on its threat and declares war on the Serbs and Bulgarians in early November. Russia declares war on the Austrians and Germany in turn declares war on the Serbs, Bulgarians, and Russians. Due to disputes between the division of territory and some border skirmishes Greece declares war on Bulgaria. After France refuses to stop its own mobilization Germany enacts the Schlieffen Plan. Britain declares war on Germany in response.
Is this a plausible scenario? What happens next? Does Romania pick a side, and if so which one, or do they wait it out to see who the winner is? With the chance to take territory from the Austro-Hungarians, Greeks, and Ottomans Italy would be tempted to join the Entente, but is still exhausted from the Italo-Ottoman War. How long would it take them to be ready? Compared to OTL, does this scenario benefit the Entente more or the Central Powers?