Then it sounds as if A-H may survive until something fractures it between Austria and Hungary, anything ranging from the losses of WW1 to the Great depression, that stress breaks it into an independent Hungary and the "Austrian" part disintegrates?
Well, we are discussing a sceniario when the Great War doesn't occur, thus the Great Depression doesn't take place either. So we have to look for other possibilities, if we want to save, desintegrate, split, smash or whatever the Monarchy. But if the July crisis still happens, we have definately more time until such problems spiral out of control.
If something breaks up the Monarchy internally, it is most probably an Austro-Hungarian breakup spiraling out of control, but in my opinion, in the end Austria and Hungary would always reach an agreement between them, since the external political environment would extremely menacing, you know... Russia. I think the breakup betwen Austria and Hungary is possible only if there's no Russia to worry about. We can play with the idea, when and how could we eliminate this menace.
But after the death of Franz Josef, what if somehow Austria becomes part of Germany, leaving Hungary with Galicia, Dalmatia and Bosnia?
Perhaps the Slovenes, Croats and Bosnia, maybe Dalmatian coast, all spin out to form a Southern Slav "Kingdom" (maybe uniting with Serbia or not). So the Slovaks and Romanians are not struggling for independence in Hungary? Or have enough "rights" to assimilate?
Desintagration would affect both Hungarian and Austrian economy pretty badly, so such a break could turn into a range of violent rebelions and/or revolutions, so it is also possible, but I doubt they would join Serbia, the Croats might even go to war with the Serbs or vice versa as a showdown between the Serb-lead yugoslavism and the Croat-lead Illyrism. Austria and Hungary might exploit from these differences.
Slovaks were quite ambivalent on these matters, since they were torn into three or four faction on this topic. Some embraced the idea of an union between Czech, Moravian and Slovak lands (Czechoslovakia), some wanted an independent Slovak nation-state, while some just wanted territorial autonomy inside Hungary, and there were the uninterested ones as well.
Romanians are a tougher subject, since there was a relatively high nationalist-separatist sense among them, but they could achieve something only if Hungary turns into a very serious turmoil, which is already a less likely sceniario.
Many of the minorities would not assimilate, I'm pretty sure about that, but their proportion would shrink compared to total population. That means most of the rural Romanian populated areas would remain Romanian, so assimilation is not the best description of this process. Maybe we could say the Hungarians "outbirth" them, thus making them less of a threat?
I would think that anything that causes the "Austrian minorities" to agitate for independence would also cause the "Hungarian minorities" to do likewise.
Ofcourse, withou doubt, but what I want to say here is that Hungary proper is much easier to hold together in such sceniario, because of the reasons, already mentioned above.