So this is just a sort of quick look at what my version of Marienburg will be like and when I start writing, I shall update this post with all the different units.
The idea is a sort of de-industrialised British Empire (3rd Empire) crossed with the Empire. The basic core unit will be a bit like an Empire Handgunner but with Bayonets as well. As for the rest, you'll just have to see... (or use the Tilean inquisition to get it out of me

)
Verezzo
Marienburg is really not that different from the rest of the Empire to warrant a book of its own.
Oh don't worry Lyynark - it'll definitely work as it's one book - infact the Imperial Guard style Commisar - the Colour Sergeant (oops

!) will really add something different to it...
Is there even any fluff to support that Marienburg would have that kind of army structure (i.e. Soviet). It's a wealthy former empire province and the only real departure from the standard empire formula would be the inclusion of a larger amount of mercenaries.
If you plan to take a vastly different path you might as well name it something completely different since it's no longer Marienburg. There was actually an appendix list in the 6th edition Empire army book which covered Marienburg, might be worth to check that out.
You wouldn't know if a certain Hobbit has the file, would you Lyynark?
It's not part of a separate rules set, you can only find it in the 6th ed army book. It's just Empire with RoR's as core.
It should be noted that the appendix Marienburg had no access to knights (and thus no Grand Master) other than RoR, which is fitting.
No - there will be no knights. However Senate Guard (there I go again

) can be upgraded to ride Barded Warhorses.