You misunderstood what I said. The Croats would most probably support the goverment in Vienna as long as they do not touch the existance of Croatian Sabor(diet) but I think they could be willing to negotiate a solution that would mirror to an extent what Scotland has today in UK. So Croats would support a general A-H diet as long as their Croatian diet remains. The Croatian diet is one of the cornerstones of Croatian identity. Among with many other things that was done in first Yugoslavia the disolution of the Croatian Sabor was one of the main things the Croatian people were resentful for.
edit@ I could actually see Czech and Croat leadership working together and conditioning their support against Hungarians with greater autonomy within the monarchy.
Ah, I misunderstood. Sorry about that. I think the Czechs and the Croats would try to squeeze any advantage they could out of Franz Ferdinand, but that the Austrians would be able (and most of all, far more willing) to offer far better terms than the Hungarian nobility.
Austria, like Russia, was a state that pretty much relied on the prestige of the army as the main reason people accepted the state. When the army collapsed, so did the state. Note that Germany survived the collapse of their army, even though their government did not.
My take on how this goes down.
Franz Joseph dies early spring 1917. Franz Joseph is crowned Emperor of Austria. The Hungarians try to push more demands as part of the Ausgleich 1917 (due for October or November, I think). Franz Ferdinand is having none of it and are actually making demands to come to Budapest and be crowned.
Both sides send out feelers. The Romanians and Serbs in Hungaria are pretty much content with getting the vote along the lines of the Austrian parliament. The Slovaks would want their own state, but realise that they cannot get it. The Croats will not support Hungary without getting its own state, which the Hungarian nationalists refuse. The Czechs want more autonomy, and is promised it. The Italians mostly want to become part of Italy, but they are mostly not consulted.
As for foreign support, Germany supports the Austrians. They will move ten divisions or so to guard the Austro-Italian border in case of a conflict, allowing the entire Austrian army to fight the Hungarians, and sending a clear message to Italy - stay out. Austria, Italy and Germany are still allied at this point, and the Italians will probably get the message clearly. Perhaps they can be bribed by promises of support by Austria and Germany for Italian designs on Albania and Ethiopia.
Serbia is pretty much international pariah since the murders of the royal family. Russia will support them, if they are attacked, but not if they start a war of aggression. They will probably, like Italy, try to gain support by France (one of their primary trading partners, and their biggest source of arms) and Russia, but will not get a blank cheque anywhere. If the Austrians bog down in Hungary, they might see an opportunity, but they will wait for a year at least.
Romania would like Transylvania, but without Russian support, it will be hard or even impossible. While an Serbo-Italian-Romanian alliance might be able to handle Austria, if it is embroiled in a civil war, German support for Austria makes the prospect bleak, especially without Russian or French support.
Russia will not support the Hungarians and would probably intervene like in 1849 if asked - they will probably not need to, but they will not partake in any dismemberment of Austria-Hungary either.
France will probably send feelers to Russia - they would be willing to support Italy in a war, but only if Russia is in on it too. They don't want to face the German army on their own, and not without the British navy to help. And the British will most likely not grant any support for French revanchism at this stage.
All of this might very well change if the Hungarians can hold out for a year. Then the small nations can claim they need to protect their ethnic comrades inside Austria-Hungary and 'intervene', but they will not attack immediately, since it will be seen as a naked landgrab and will yield them little to no support.
As for the actual war, how does it start, and how does it pan out?
Probably, there's some violence in Banat, Transylvania, Slovakia or Croatia. Franz Ferdinand uses it as an excuse to send in the army. The Hungarians protest, but the Austrians claim they are just restoring orders.
The Hungarians mobilise the Honved just a bit too late, they are still trying to get international support and get the minorities aboard and do not want to be seen as the aggressors.
The Austrians, with German support, have more men, more guns, more industry and most likely they'll also win the international propaganda war. While there might be some stirrings in Congress Poland (supressed by Russian troops) and some romantic ideas in England about the 'brave Hungarians', the fact that Hungarian is a minor language very unlike the most spoken ones in the world, while German and French (lots of Austrians, especially among the upper classes, speak French) are big ones gives the Austrians a big edge in the propaganda war.
As for the fighting, it depends a lot on the artillery. 1914, the Austro-Hungarian army had 1870s and 1880s vintage artillery, with modern pieces in the pipeline. OTL they lost a lot of their trained pre-war gunners in Galizia and Przemysl and when their artillery got new guns, the well-trained men to man them were gone.
Austria, especially with Bohemia-Moravia, is far more industrialised than Hungary. It is quite possible that the Imperial and Royal Army and the Austrina Landwehr have modern artillery, while the Honved still have the 1870s and 1880s artillery, which will make the Austrian forces far superior.
The Hungarians will have better cavalry, a good force in the Honved, decent support and probably a lot of nationalist volunteers. However, the Austrians will probably have secured Slovakia and Croatia before hostilities start up, and the Hungarians will be isolated without a port. They have no access to nitrates for ammunition and no industry to produce more arms and ammunition. While they may be able to secure an area roughly equilent to ethnic Hungary, with fighting going on in Transylvania, they will not have the resources to hold it, since they need to fight on two or even three fronts (and guarding against the Russians to avoid a repeat of 1849).
I foresee the Austrians, suffering large casualties learning modern warfare will push over the Hungarian plains to Budapest within two months, then be held up fighting for Budapest for another two months, as Budapest falls, so does Hungary, having no resources and no support to keep fighting.
The aftermath? Some kind of centralised federation. All regions are allowed their own parliament that handles police, local taxes, schooling, social affairs etc, while the central parliament in Vienna handles the army, the federal taxes, the navy, the foreign policy and federal authorities and entities such as state-owned companies, railroads and the Donau, pensions, federal police and of course foreign policy.
The army has gained a lot of prestige defeating the Hungarians, most ethnic minorities get a bit of what they want, the Hungarian tenant farmers (semi-serfs) get the vote, the Hungarian parliament is downgraded to a regional one having no say in foreign policy and other federal decisions.
