As for jolting the nationalist ideal, don't you remember what caused WWI in the first place?
The use of Ancien Regime was an anachronism for Versailles being a rather belated continuation of the Congress of Vienna mopping up after Napoleon. It's what the Congress could have been like if the victors were aware of the tactical advantage of small nationalistic states as opposed to 'stable' ambling empires.
I can see your point, but I'm not much of a backer of the alliance/military/expansionist theory of the cause of WWI. I'm back the economic/industrial theory of the cause of the war. As I've said before I'm not a conservative historian. The visual nationalism was tacked on when Europe went to war (as happens when most countries go to war) Germany wanted to smash the protectionist elements of the British and French Empires (hence the dry run Morrocan crises) and re-order Europe to establish a customs union. Unfortunately it had to violate the territorial integrity and independence of it's neighbours to do so. The main threat to Germany's position as the economic powerhouse of continental Europe was Russia. Germany played a game of 'smoke and mirrors' to portray Russia as a potential agressor to it's sleepy Austrian allies. It also explains the terms of Brest-Litovsk. It also ties in with why Russia became the first communist experiment and not say, Germany as Marx had predicted. There are echoes of 1848 in there too but it gets rather long winded.
And then of course there's Poland 1920 as you've pointed out. The men who said 'join, fight and turn on Russia' after the defeat of Germany twice last century were not as daft as history has often made them out to be. Thank goodness we didn't do that of course
Just a few things.. while I agree that Nationalism was often a smoke screen for various other political and economic interests of the German, British and French governments it should not be forgotten that nationalism was also the major threat to European political stability in the period before 1914. Especially where the war started in Balkans where the Romanovs, the Hapsburgs and the Ottomans had been disintregrating in power and influence leaving a huge power vacuum in its wake mainly taken up various Pan-Slavist groups (in Serbia) and expansionist and feuding nationalisms (in Bulgaria and Greece; see Second Balkan War), the Nationalist coup in 1908 which overtook the Ottoman Empire should not be forgotten. Not that this alone a problem for the Balkans but across Europe, none of the states that went to war were really particularly stable and it should not be forgotten how nationalism divided any potential opposition to the war, especially within socialist circles (Germany being the prime example of this).
Britain: Had a potential civil war about to break out over home rule in Ireland
France: Is France. And the Third Republic was hardly a beacon of stability
Russia: Where to begin? Increasing tensions with ethnic minorities on their borders, widespread violence following 1905 and the weakening of nationalism (which was still very much associated with Orthodox Christianity at this point btw) following the defeat to Japan, an antiquated 18th Century political system which still relied on the Tsar being god's representative.. who no-one respected. Also the Backwardness of the country created a much large anti-establishment intellectual class bitter at the failure they saw around them. Add to the mix then increasing industralisation (and also 'workers soliditarily') between 1900-1914 which was mainly done so Russia could maintain being a power. But the contradictions quickly imploded within themselves.
Italy: Unification was mainly due to other powers and the shifting alliances of mid 19th Century Europe rather than any idealism and anyway "things only changed so they could stay the same" (From The Leopard) and a great deal of disillusion set in after 1871 after the clear monarchial nature of the new regime was apparent. Then add up 30 years of unification not alliveating any of the goals it set out to 'do': didn't fix the backwardness of the South, didn't create a much more efficient or loyal civil society. In fact Italian society was as corrupt as ever and ergo it isn't surprising that by this point clevages of hard left and right begin to emerge with a very strong anti-establishment nationalism of which D'Annuzzio was the best representative (and Garabaldi 2 generations earlier) which eventually led to the coming to power of Mussolini.
Germany: Socialism and Industrial tension contributed to problems within the 'Prussian' Reich mixed with an expansionary foreign policy which required a strong Austria; in both cases Nationalism was a major threat (except non-German nationalism o/c)
Austria-Hungary: Well we all know.. and there is no need to go into that. Needless to say it is clear what ideology Gavrilo Princip had (though the war was inevitable by that point).
Basically my point is here that nationalism was 1) a unifying agent between each nations elite and their people (which would soon be replaced by socialism and fascism in places..) as long one could portray the national cause as 'right' and 'correct' thus 'poor little Belgium', 'Russian Agression', even Pan-German sentiments were stoked up while all the time believing the war would be over soon and 2) the major opposition movement towards de-stabilizing Europe and destroying what was left of the Feudal order. Also to Mention that parts of the European Economy was getting increasingly integrated.. the period of 1870-1914 is the 'first great age of globalisation' after all and where that happens there is both increasing cultural dominance by the more powerful societies and resentment and opposition to that.
Of course it should be pointed here that the Congress of Vienna had been restore the power of Merchantilist era Monarchies across Europe following Napoleon, by 1914 they had disappeared in France (Revolutionary legacy, I think the French still believe they invented Poltics) and had weakened with pressure in most of central-eastern Europe (and had dissapeared in a couple of minor places like Switzerland) but by 1918 it was all over.
* - Okay, Okay, I admit British exceptionalism, though they did lose Ireland thanks to the war, which is something which seems to never be mentioned in British history much.