Thanks for the comments everyone.
If I may ask, did these reforms also affect the existing terminology and status for administrative units in the various lands of the Austrian crown ? Do they follow a new, standardized hierarchy and naming system, or have they just modified the old ones, at most ?
My general idea was that the old administrative boundaries would continue in a sort of theoretical sense even as the actual apparatus attached to them is redistributed, sort of like traditional counties in some parts of the UK. Nobles need their titles after all, even if those titles start to mean something different.
Now that's a fascinating variation on the Usual 'United States of Greater Austria' theme that usually emerges in ATLs.
In part it was inspired by criticism of that idea by people who have pointed out the sheer level of intermixing across the empire, at least in major cities.
I concur. A thoroughly enjoyable update. Rudolf III. strikes me as inadvertently developing into an ATL version of Joseph II. (even the whole Erzkönig idea reminds me of how Joseph was commonly nicknamed "the king with the hat" or "the bureaucrat-king"). I hope the monarchy continues the course that he so stubbornly helped set up. The sociopolitical subtext behind the reforms of the musket and rifle regiments and the Gränzers are cunningly ennacted policies. Donauland/Danubia has a nice ring to it, though I hope that the traditional Austrian lands will still be called Austria even after the realm rebrands its official international name.
"The king with the hat", I like that. I haven't seen Grenzer spelled that way before, where does it come from?
Is the version of 'modernized Latin' the Austrians are using in the rifle regiments have any relation to Novalatina (IIRC) in the UPSA? It seems likely that the Meridians would have at least heard of the Austrian attempt to use a revived Latin when they're creating Novalatina. It also seems like the kind of thing that could cause friction with future Diversitarians. If you're a Diversitarian with a reasonably paranoid outlook, a state using the same language as the hated Societists is bound to raise red flags. Excellent update.
I was hoping someone would mention that. I was going to delve into it more in the update but I thought I'd just leave it there as a hint. It is actually parallel evolution (the Austrians in OTL used Latin for some things for the same reason) but all the things you raise will be unintended consequences.
For instance, how would a case between citizens of two different nations be decided? Would it be sent directly to the federal courts, or to the "other" courts? Or would it be heard before a mixed panel of judges from both nations?
As you point out, this is something which in theory is constitutionally sorted out beforehand, but in practice will be more difficult. The idea is that if a Hungarian wrongs a Romanian, the case will be tried in a Romanian court, and if vice-versa, in a Hungarian court. But most cases are more ambiguous than that, leading to either mixed setups or, often, appealing to the 'Other' federal courts as a neutral arbiter. An example, as you imply, of how what looks like a neatly worked out system on paper may turn out to cause more headaches in reality.
Also if I have understood it correctly, the "others" have no diet of their own to make laws concerning them. Are the directly subject to the federal government?
Yes, though they would have a federal parliamentary committee set up to look after their interests and a court of highest appeal. As you point out, they do get a bit of a rawer deal than the four main groups.