How do you get the historical data for your games? Are there some existing databases for provincial economics and assessed skills of generals and so forth, do you go thru huge piles of books and archival records to create your own assessments, or just guess something and wait for possible corrective suggestions by users? Assuming not guessing, do you credit your sources as in scientific articles, if so, where? If you do your own research, how far is it from being a publishable scientific article, eg. "economic development accross the world in 1444"?
Do you have separate coders and historians (and of course artists and marketing and whatever) or "just" coders with historical interest?
I haven't noticed (perhaps my bad) competition in the grand strategy market with similar level of detail. Is it just due to the size of the market, or are there relevant copyright/plagiarism/etc issues? Eg. if someone was to produce a game in which a 1444 map would look a lot like EU4, could Paradox sue them?
Honestly a lot of it is from wikipedia and other online sources these days (the HoI oobs rely heavily on Niehorster etc.). We do have a research library, and add books to it when we need some more in-depth info on a specific issue. But the historical facts are only a starting position, before balancing starts. So with a general for example, we would look them up either online or in the library, and see what is known about this person. If they were famous defenders they get a bonus point to defense skills for example, but if that turns out to be too powerful, we might remove it again.
While I think our games have started to edge close to being a very comprehensive picture of the world at the various starting dates, I don't think it would stand up to scientific scrutiny, because of the balance concerns overriding strict historical accuracy. Our game mechanics are also always, to a certain degree, abstractions and generalizations, and historical precedent is a set of extremely messy specific cases. Every game has their one famous edge case that is historically correct, but runs counter to the entire underlying structure and mechanics of the game (the Duke of Normandy for CK3 being both King of England but a vassal of the King of France comes to mind) We also don't usually update our databases when new research becomes available, we only really update when we are reworking some content anyway.
As for historians, we have both. Our Content Designers and Game Designers are the people mostly concerned with the historical starting setups, and quite a few have history degrees (some even PhDs), but we also have a lot of people in other disciplines who have a keen interest in history. We don't have any dedicated historians whose only job it is to research stuff - that is generally done by the CDs and Game Designers.
The reason why there have been few successful attempts at Grand Strategy Games outside of Paradox is primarily, I think, because making a GSG is not a small task by any definition, and it isn't very scalable. You can't really make a "small" GSG - you'd just make a regular strategy game. Game Development is a lot about risk management, and there are few companies where you can come and say "hey, I would like to pay a person half a million Swedish Crowns per year to figure out what the political structure of central India in the 11th century was like, down to the level of individual baronies. This isn't going to make us any money directly, but we can build a game on that research. That's gonna take a lot more people and several more years, but it'll be good, trust me, once we have added the other 20 game mechanics that all have to be there to make the game work." That is a big outlay of money with no guaranteed return on that investment. Paradox mitigates that risk by having a) a bunch of people who have shown that they can figure out the political structure of southern India in the 11th century and b) a large database where that work was already done from previous games.
I'm not going to comment on the legal issues because copyright law is difficult at the best of times and a lot would depend on the specific case.