I did not write that nationalist myths are irrelevant. I wrote that a multi-etnic empire, which had to keep nationalism in check for its own survival and for the safety of its subjects, did not have to bow to them. The Austro-Hungarian Empire included Serbian and Muslim subjects. Muslims were being killed by Serbs in recently-annexed Macedonia. Honouring the myths used by radicals to fuel the flames of hate was not the way.
The Austro-Hungarian empire was not as sensitive to the dangers of nationalism as you are arguing. I think some people could see the problem spiralling out of control - Franz Joseph and *perhaps* Franz Ferdinand, who both had particular animus against both German and Hungarian nationalism. But the very existence of the dual empire was in itself predicated on Hungarian nationalism of what became a particularly repressive kind. If the Austrian ruling class could see this, the Hungarians clearly couldn't. And, passive as it was, the Austrian side of the empire continued to maintain the dominance of the German language to an unbalanced extent - to the extent that grammar schools could bring down the government. It was, I agree, much better than many of its peers - and we tend to look back on it with nostalgia, as did its subjects later - but it was not the perfect multi-ethnic state. Multi-ethnic by accident, not design. And its development was patchy, geographically if not ethnically. The south slav states Bosnia and Dalmatia (with Galicia, a less recent acquisition) were the poorest parts.
That is by the by, just posted because I like robust debate! :-) Among the men who killed Franz Ferdinand was one Muslim. To this little group, religion was infinitely less an issue than the question of foreign repression. To ethnic Serbs (as we are not talking about the State of Serbia), the Kosovo/Vidovdan meme became an anti-occupation one - whoever the occupier. It was not anti-muslim. They grossly and fatally overplayed this when the south slav state became a reality, in assuming that others (the Habsburg slavs) would take their national day to their hearts, but part of the reason why it became an issue for the Catholic slavs was the death of Franz Ferdinand on that day. In 1914, the day was far less charged to Austrian subjects. In 1921, actually, the one ally Serbia had in approving the Vidovdan constitution were the Bosnian muslims, because it was a secular constitution - which protected the muslims against the near-pogroms taking place in parts of the countryside.
I'd say again, though, that it doesn't matter. By refusing to side-step the day - or even to try to turn it to their own benefit by emphasising that Bosnia had joined Austria in escaping the Ottoman empire - they lost their crown prince.