Did the KuK units in Hungary consist primarily of Hungarian soldiers?
Essentially, the millitary system of the time was based on standing garrisons forming the nuclei of territorially based reserve formations. Since standing army recruits, upon release, generally returned to heir home farm this meant that in practise most recruits were attached to territorial garrisons that were near their homes, in order to ensure unit continuity. Of course, frontier regions had a higher concentration of regular army garrisons, and AH and Russia, as multiethnic empires, made some effort to counter the tendency to serving in the home province garrison would not be followed 100% in regard to troublesome national minorities (primarily Poles in hte case of Russia, South Slavs and Romanians in the case of AH). However, Magyars, as far as I could see, were not given this "special treatment" by the KuK from the 1880s onwards, and almost all regiments in garrisoned in the Hungarian heartland had Magyar as their primary regimental language.
Just for Germany:
ITTL they estimated that Russia in 1917 was too strong for the Schlieffenplan. And even in German circles A-H was often called the "assortment of decaying nations around the Danube".
The German General Staff did asses, *in 1912*, that Russia would be capable of rapid mobilization AND massively expand it's forces by 1917. Their conclusion however, was not precisely that Russia could not be beat, but that it could not be beat in a short war (ie; utilizing the Schlieffen Plan)- which was the only type they were prepared to consider. If the alternative is permanent subordination to Russian goodwil, they might be prepared to chance a long war - if they fought they could win it, or if they found any other course of action intolerable. millitary, and civil, leadership, has not historically always acted rationally in this respect.
I would note that the German General staff erred in this assesment. On the one hand, Russia was able to mobilize far more quickly in 1914 than the German General staff assumed it would, launching a numerically credible invasion of East Prussia and East Galicia within 10 days of mobilization in support of their allies and arguably burying the Schliffen plan. OTOH, their performance vs the Germans sucked wet dogshit, not only in 1914, but also in 1916, after they should have been past their learning curve. So it is by no means clear that Germany could not defeat Russia in a head-on clash in 1917, even if they are fighting alone, with one hand tied behind their back facing France defensively. Of course, if they Choose to gamble on Schliffen and write off everything East of the Oder (in their own minds, not neccesarily in reality) while they take Paris, The war may well be over within a few months.
The question is if Kaiser Wilhelm's personal friendship to FF would override diplomatic/strategic necessities. The sensible thing would be to accept rump Austria, while satisfying Russian and Italian demands. This would basically give Germany a free hand in the West.
Italian demands would essentially be the left bank of the Isonozo, Istiria and Trieste. The latter is an economic sucker punch to rump Austria- it is one of the four primary gateways to Southeastern Europe. Italy would also however seek Dalmatia and perhaps South Tyrol- if FF cedes the former any Croat loyalty to the monarchy goes out the window, and if he cedes South Tyrol ethnic Germans in Austria (and germany as well) will be outraged. FF's throne does not look too stable at that point.
Still, Italy's demands can be satisfied in a way that transforms Italy into a dependent ally against France.
Russia's demands, however, cannot, or not fully. In addition for Bosnia or parts thereof to the Serbs (more trouble for FF with the Croats) They will want Galicia and Bukovina (north Bukovina to keep, south Bukovina to trade to Rumania for concessions) which means coming to an accomodation of sorts with Polish national sentiment. That in turn means trouble for the Germans in Posen. Russia mooted several times prior to 1914 restoring the autonomy of the Kingdom of Poland. It was not merely Nicholas II's obstructionism, or the fear of encouraging demands for autonomy from other subject nationalities. It was also the knowledge that this would be tantramount to a declaration of permanent hostility against the other partioning powers.
In any Event, Russia will never permit Germany a free hand in the West. At best, if Germany, as AJP Taylor describes Russia's WWI war aims "Leaves Russia alone to do away with Austria", Germany can hope for a return to the pre Berlin congress friendly attitude of Russia and perhaps an anti-British alliance. But the dual alliance is too firmly entrenched in Russia's diplomatic culture by this point to ditch. Russia may not back a war of revanche, may even offer reinsurance treaties to that effect in return for a satisfactory partioning of Austria, but it will always keep the French alliance as a way of constraining German power- and post Bjorko, Germany will have no hope of any other outcome.
Given the trend of faster Russian industrial and demographic development, and the growing density and carrying capacities of its railways (The millitary role of Motor transport in 1917 will not be apparant but it will increasingly reduce Russia's disadvantage in concentrating it's forces and supplying them), doing away with AH deprives Germany of a key counter to Russian power in a way that they have no gurantee can be replaced by Hungary (whose sepratists tended to want out of Great Power politics and might prefer a neautralist Russo-German stance) or Italy (Who even in the heyday of the Triple Alliance never seriously contemplated sending troops to fight Russia). Nor is there any Gurantee that a rump Austria betrayed by Germany would be interested in bunding itself into an alliance with it, or honor it in case of a future conflict (Which lmakes annexation more attractive - but has its own internal problems, and it is by no means clear that the entente will sanction a non voluntary takeover, particularly of "Non German" lands - which may include any part of Bohemia and Moravia as Russia and France define it).
What all this means is that doing away with AH leaves Germany apparantly dependent on Russian goodwill ("apparantly" because Russian millitary capabilities remain abyssal, far worse than Germany estimates. It is by no means clear to me that Germany will necessarily lose a war if it chooses to fight and avoids marching through Belgium) right now, just as they were in the post Napoleonic world, rather in some nebulous future. That is a grim dictate which Germany's decision makers will find hard to swallow, just as Britain found complete vulnerability to the U.S, and dependence on its goodwill hard to swallow. The diffirence being, of course, that Britain had several generations, and two wars in which the U.S supported them to get used to the idea.
This hypothetical grand coalition against Hungary simply won't happen.
What I described is less of a Grand coalition against Hungary and more of a pit filled with pitbulls, each circling the two biggest mutts and manuvering for advantage as spectators egg them on, and offer inducements to achieve a given configuration suiting their bets. In other words, depending on cicumstances, a given faction within a given nationality might switch support from Vienna to Budapest or vice versa, while seeking external backing from Rome, Moscow or Berlin, all in an attempt to gain advantage. If the Empire (Or Hungary) seems strong they will manuver for a better place within it. If weak - for a chance to break away and secure territory. If fallen - to defend or attack competing nationalities. And of course the relative balance of power between factions in a given nationality will change as well as circumstances do.
Many Romanians will definitely try to take a third option (complete separation from the monarchy and union with Romania).
The question is whether Romania is prepared to openly back Transylvanian rebels if Russia says "Niet" and Bulgaria growls at it from the South. I think the answer is that it isn't. Supplying weapons and allowing volunteers across the border, yes. Invasion, no. Not unless FF takes Budapest and Russia writes the Hungarians off in the context of a war it still seems to be winning (or which AH seems to be losing, which is not quite the same thing).
However, from an objective point of view a ROmanian who is reblling in favor of Bucharest or Vienna is all the same to Budapest- it's the threat to the Hungarian nation and a resource drag.
And a minority of Slovaks may back Hungary.
The point is that some ethnic conflict will take place, and that FF will be blamed for ALL of it by nationalist Hungarians unless he openly disavows those protesting Maygarization. WHich he won't, unless Magyar nationalists make concessions favoring. Which they won;t because they don't trust him -especially after Croatia. I do not see these clashes/uprising as being millitarly significant in regards to Hungary's survivability in and of themselves. They will be significant (especially in the Banat and Burgenland) in regards to constraining negotiations and attempts at compromise between Vienna and Berlin.
As for Serbs, there is 0 chance they'd side with FF, let alone petition him for anything. In the event of any kind of crisis between FF and Hungary, Serbs will side with the Hungarians - that, or go their own way. And that goes for all ethnic Serbs across the monarchy, not just the ones in Banat. Serbs in the Croatian crownland (who, at the time, made up 25% of the population) will present a major stumbling block to the Croats' hypothetical pro-Vienna revolt.
Here I will dispute you. Hungary was able to play divide and rule with the Serbs in Croatia and the Banat for a long time, but even in 1906 that policy was petering out and the dominant Serba and Croat factions within Croatia, the Banat and Dalmatia were pro serbo-croat collaboration (even if tactical). That does not mean that Serbs and Croats won't clash once FF ends up backing the secession of Croatia from Hungary (and merger with Dalmatia)- but those who clash with the idea of a Croat dominated kingdom within Austria will be looking to Belgrade, not Budapest.
Germans in Hungary were generally loyal to the idea of a Hungarian nation, many of them were rapidly assimilating and some of the biggest Hungarian nationalists around were actually of German origin. So they won't be propping up an anti-Hungarian coalition either.
It depends where. In inner Hungary Germans seemed to have generally subscribed to the Idea of a Hungarian nation and were in the process of assimilation post 1867. In Burgenland, the Banat and the Saxon region in Transylvania this does not appear to be the case (unless you rely on Wikipedia).
In other words, the balance of power might still be leaning towards FF's side, but it's not
overwhelmingly on his side. It won't be a quick and easy curbstomp of the Hungarian separatists.
Disputes about sentiment aside the overwhelming issue is one of millitary organization and freedom of action. FF enjoys an organized army and the mechanism of conscription. The Hungarian palimant does not. organizing an army out of the Honved, volunteers and defecting individuals/units takes time. And of course- in a prolonged war Hungary's lack of heavy industry will tell against it. So I would argue that even if Hungary's minorities are not supportive of FF, it is a curbstomp - if the the international situation + situation in rest of the Empire allows him to deploy his army freely. In this scenario, it does not.
Now, on to Germany's reaction. Crucially, Germany thought Dualism was the best thing since sliced bread and greatly respected the Hungarian ruling elites. Berlin would see FF's actions as downright idiotic, and as a gross lack of respect for Berlin's own preferences.
Exactly - that is my take as well.
So I'd say option C) is very likely.
The question therfore is whether Hungary would be prepared to sign on to the revived Triple Alliance in return for German guranteees of holding Vienna off - and whether Cisleithania can hang together and/or face off even Italy and Serbia alone once the loss of prestige rocks the legitimacy of the monarchy and triggers secondary Czech-German, Ukrainian-Pole, German-Slovene and Croat-Slovene-Bosniak-Serb conflict over the future (federal?) shape of the rump Monarchy.
D) is not too far off, either. You'd have a situation where FF's very name evinces eye-rolling and spitting in German ruling circles...and the idea of a German-managed partition presents many potential advantages. Hungary can be kept in the Triple Alliance.
Can it? That is the question. Hungary would want gurantees from a Great Power until it can organize it's own armies to see off Serbia and Romania. But once it's organized it can deter them without Great Power support. It would really only need to be in the Triple alliance to prevent a Habsburg (if CisLeithania hangs together) reconquest. And unlike AH, which had great power ambitions in the Balkans which clashed with that of Russia, I can't see and independent Hungary being concerned with Russian schemes in the Dardanelles. So would Hungary really sign on to a semi-permanent alliance with Germany the way AH did? Would Germany expect it to?
The Austrian and Czech lands can be annexed to Germany
I can;t see Russia, Or even France and Italy for that matter, counterancing direct annexation of the Czech lands. Continued rump Austrian control of Bohemia, yes. Pan-german Empire control - no. It would empower Germany too much, and the Czech would resist sufficiently to make it an international issue. Perhaps the Sudetans might be where the compromise is drawn.
- or, if they stay nominally independent, turned into an appendage of the German state.
The question is how. FF is not going to be happy about being "stabbed in the back" by Berlin. He will seek greater freedom of action, not less. How can Berlin ensure he does as he is told, without risking that he will seek asssistance in preserving his remaining independence from Russia-France? Diplomatic turnarounds happen. To be sure Ethnic German sentiment, and that of many of the (ethnic German) millitary officers and officials will favor close collaboration with Germany. But the remaining slav's who will still be a slight majority (if a full carveup of CisLithenia took place), will oppose such subordination.
My sense is that Germany will be pushed into a second dilemma- does it cash in on what Russia, Italy and France are prepared to let it have (OTL Austria + maybe Sudetans) and leave a rump Czechia as a economic sattelite and a millitary thorn, or does it seek to subordinate a Bohemian-Austrian-Croat Empire whose Emperor has his own mind.
Of course, it's also possible that Germany would swallow the bitter pill and decide to back FF. But like I said, there would be much eye-rolling, and spitting, and gnashing of teeth.
My take as well. Which would presumably correlate with greater willingness to seek D) in the context of a compromise peace if the war stalemates.
The new King of Hungary would likely be the then-Count Palatine, a scion of the Hapsburg Lorraine dynasty.
The only way I can see this happening is if the divorce is semi-amicable/mediated by Germany with Hungary remaining part of Triple Alliance