I wrote a scenario around this on another board at one time.
When it comes to foreign relations, Russia is the big question. Russia sent troops to subjugate the Hungarian revolt 1848 - an ethnic rising against an old multi-ethnic Empire would spell bad news for them. Finland and Poland want to be free, and there's nationalistic tendencies in Estonia, Latvia and Lithuania. While the relations between Austria and Russia soured over the Balkans, Russia attacking Austria in support of an ethnic revolt is inplausible.
Serbia would certainly attack if she thought she could get away with it. The 1903 murders of the royual family and the coup that went with it showed that the Serbians had little concern for the diplomatic system (they suffered tremendously diplomatically for it). But without Russia, they will probably not dare attack on their own - after all, the Bulgarians want Macedonia back, and the Greeks are eying the same territory, and they are busy rebuilding after two ruinous Balkan Wars and absorb their substantial conquests in those wars.
Without Russia, Serbia will probably not act. A provocation could cause war, but if Austria remains on the defensive, Russia will probably not intervene.
While Romania would like to get their hands on Transylvania, their monarch and his government are German- and Austrian-friendly - OTL it took the Brusilov offensive and Austria-Hungary looking ready to collapse, and the Austro-Hungarian forces engaged both against Italy and Russia for the Romanians to enter the war.
Italy might want a piece of Austria, but they are formally allied, and attacking an ally is a big no-no at the time.
Austria-Hungary is Germany's closest, oldest and best ally, and a useful tool for keeping catholics and the Balkans in order. They will NOT attempt to disassemble Austria-Hungary. Rather, they will offer troops, supplies and diplomatic support. The Germans will let the Romanians and Italians know that any aggression against Austria in her time of peril will be considered aggression against Germany. The Austrians will probably reject direct support out of pride, but German troops might take up positions along the Austro-Italian and Austro-Russian border, and volunteers and supplies as well as observers and staff officers will be sent.
France will probably support the Hungarians, at least diplomatically - they had a soft spot for subjected people in eastern Europe/potential allies against Russia/Germany/Austria, which is why they supported Poland 1863 - but they will not move without either Britain or Russia. They know they cannot take on the Germans on their own.
The Hungarian people were voting with their feet at the time - 1880-1914, more than 7% of the Hungarian population emigrated, so I doubt the nobility that control the Hungarian parliament will find much support among the populace, especially if Franz Ferdinand offers male suffrage. The Croatians and Transylvanian Romanians would not support Budapest - they did not 1848. And I doubt the Slovaks will either. All of them have more to gain by supporting Vienna.
I suspect a 3-month campaign before the parliament surrenders, 2 of those months being a siege of Budapest.