or simply because a monarchy can't allow itself to be liberal above a certain level without endangering itself :
Well, AH is a peculiar case because the construction of the state as a sort of "dynastic federation" lacks functionality once the monarch is a failure, but you should note that it is the liberal monarchies of Northwestern Europe whose thrones are still occupied in 2012.
is it possible that:
1) the Bosnian and possibly the other South Slavs (Croats ans Slovene) will rebel
2) the Russians and the Serbs will intervene on behalf of their endangered brothers
The probability for this to occur this way round is not that high. The monarchy in 1914 lacked really strong truly separatist movements which were in the position to start such a rebellion. The troublespots were the Czech-German differences in Bohemia (less so in Moravia) with both sides unable to really get anywhere; and more subtly, the patient and stubborn way the Hungarian Government expanded their autonomy punctually in each year ending on -7.
The Slovenes might become vociferous (for the first time, IIRC), if FF goes for a Southslav crown, but leaves them ("Krain") under the rule of Vienna.
The wider question is if Ww1 was a fluke or there were wider trends that couldn' be really avoided
WW1 certainly was no fluke. With hindsight, I would say that the chances for a major WW1-style European war in the 1910s was at 80% or more, which doesn't mean that it is absolutely unavoidable.
And, more so, it is not a given that Austria-Hungary plays the key role in the events leading up to it.
IMHO, if FF rules Austria-Hungary
and keeps to the conviction to evade war if possible, Russia becomes the top candidate to provoke war unless for some reason decapacitated (revolution, war in Asia). In that case, Germany moves to the spot, unless for some reason liberalising (Wilhelm too heavily involved in some kind of scandal, 1917 elections leading to a Reichstag ceasing to accept the Kaiser's role). In that case, nobody is really left in Europe who would be in the position.
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On languages: actually, I am not of the opinion that German as a lingua franca needs to be strengthened. The monarchy's economical, political and cultural elites knew German and used it. Measures of that kind are rather more important in order to show the other Völker that the arrangement is a quid pro quo.
And, it is doable. Although under different circumstances, and reluctantly so, my great-grandfather learnt Czech in order to keep his job in the 1920s. Compulsory learning Czech as a foreign language at school despite living in an area where the only Czech speakers were newly incoming Government officials didn't hurt my grandfather either.