Lets make some things clear here, ok?
The Second Reich (Imperial Germany) did not have designs on German-speaking parts of Austria and Bohema-Moroavia before ww1, and victorious, they will not have designs after it either.
Austria-Hungary was Germany's only larger, reliable ally and the excellent tool to keep the Balkans in order. The rulers of Imperial Germany did not want more catholics in Germany, that was why they did not annex territory from Austria 1866 and why they sought an alliance immediately afterwards.
Germany's foreign policy after Bismarck's retirement was erratic, and they had problems keeping friends. They knew this, and they were in no way prepared to sacrifice the only real friend they DID have.
Imagine how it would look in the rest of the world - to the Ottomans, for example. The Germans are trying to make the Ottomans a nice ally, investing, helping them reform their army, and suddenly, when there's an ethnic rising in Germany's oldest and most reliable ally, the Germans side with the revolters instead of the legal goverment and the Emperor?! And ATTACKS its ally to annex territory from it? Germany would lose all international influence immediately, and they know it.
The Germans would offer troops, the Austrians would refuse it out of pride, but they would be grateful for the Germans warning the Romanians, Serbs (if there is a Serb state after a central powers victory) and Italians to stay out. Supplies and a cadre of observers and volunteers will be sent, with full diplomatic support.
The Hungarians will find themselves up shit creek with a leaking canoe and no paddle. Their population is smaller, their economy weaker (Austria and Bohemia-Moravia has most of the industry) and their own subjects are not very enthusiastic. Romanians in Transylvania, Slovaks in Slovakia, the small pockets of Germans in western Hungary and Slovakia and Transylvania and the Croats in Croatia will all side with the Emperor against the revolt. Like 1848, the Hungarians will have to fight their own rear as well as the Emperor's troops.
Also, before 1914 the Hungarians themselves were voting with their feet and emigrating in large amounts. 1880-1914 about 700 000 ethnic Hungarians emigrated to the US. 1910 about 10 050 000 prople spoke Hungarian in Austria-Hungary. That is almost 7% of the people leaving!
The Hungarian nobles kept the vote from their subjects by gerrymandering and land ownership requirements and the Hungarian people resented that almost as much as the Croats, Slovaks, Romanians and Serbs in the Kingdom of Hungary.