Certainly the Austro-Hungarian Empire was not the perfect multi-ethnic state. Neither the Roman Empire nor the current United States of America are examples of perfect multi-ethnic states because such thing does not and cannot exist, being human nature as it is. Different ethnic groups can live more or less peacefully in the same country, with each group trying to get privileges for themselves or they can butcher each other. In 1914 the Austro-Hungarian Empire, including the Kingdom of Hungary, belonged to the first category, the Kingdom of Serbia, which had made a cult of nationalism, to the second one.
The one token muslim Bosnian in the team of murderers was merely there to make more difficult for Austrian investigators to pick the scent trail and follow it all the way to Belgrade. It was a Serbian nationalist plot. What killed Franz Ferdinand and his wife was Serbian nationalism, not some coincidence in the dates or negligence of Austrian authorities in paying attention to the anniversary of a battle.
The importance of the date, St Vitus Day, the anniversary of the Battle of Kosovo, has been overstated for two reasons
1. Historians love that kind of coincidences, it's the equivalent of astrology in their trade and makes things more interesting for readers. I have read in a book published by a British historian (not Mr. Clark) that Franz Ferdinand and Sophie were murdered on their wedding anniversary. They weren't.
2. Serbs, and people who are glad to accept their narrative, can use the coincidence to make the visit of Franz Ferdinand to Sarajevo look like a "provocation". This way they can shift the blame to Austrian authorities and create a smoke-screen around what happened on 28 June 1914 in Sarajevo: a dirty, treacherous double murder in cold blood.
If Franz Ferdinand had visited Bosnia on 24 June, 2 July, 15 March or 28 February, there would have been a murder attempt. And if he had avoided visiting Bosnia altogether, sooner or later a Serb would have traveled to Austria and tried to kill him.
If Serbia is "hold up as a perennial special case of particularly obnoxious nationalism", the reason may be the behaviour of Serbian authorities. For example, inaugurating a monument to the terrorist Gavrilo Princip in Belgrade on the anniversary of the double murder he commited (28 June 2015). The president of Serbia, Tomislav Nikolic, several ministers in the Serbian government and a band of music of the Serbian army attended the inauguration, a bishop of the Serbian Orthodox Church blessed the statue. The president Nikolic said: "Gavrilo Princip was a hero, a symbol of the idea of freedom, the assassin of tyrants and the carrier of the European idea of liberation from slavery".
http://www.dw.com/en/gavrilo-princip-assassin-who-sparked-wwi-gets-statue-in-belgrade/a-18546305Cristopher Clark's book is excellent and "does not read history backwards". It reads history forwards, from the murder of King Alexandra and Queen Draga in Belgrade 1903 to the start of WWI. If it has been criticised is, I guess, because it shatters the myth of "brave, little Serbia attacked by the Austrian bully" and "gallant Serbia, defender of Christian civilization in the Balkans".Obviously Serbs and their allies do not like it, neither people that wish to agree with the Treaty of Versailles in blaming exclusively "Germany and her allies" for WWI.