Wow. A post in a thread that hasn;r seen any action in 1.5 years...
as for source materials, for CORE1 we used all sorts of stuff. Alot of it was good old fashioned books (either in dev's personal collections or from libraries), "reliable" web sources (mainly university collections on line of documents, maps, etc), and personal research. We used to have a compiled list of all this, by sources available by each developer, but it has passed into Internet heaven.
Any disputes about information, or analysis, would be discussed not only among the dev's, but also in public discusion arenas such as here. We generally already had a general idea about things, so we could reach a consensus about specifics where deemed appropriate. What happened, more or less by default and by "evolution," was that we all had a "working knowledge" not only of history but of game play mechanics, and each developer had his/her own area of specialty (Naval, political, economic, etc) and had the lead for each of those areas. Given our base knowledge, we could fairly quickly and easilly recognize when something was "off" in terms of history.
There was/is no main source per se. We all use a number of various sources, from books and articles to government archives (on line and off line), to well researched web sites (there are a few very good ones out there), to personal investigations (interviews, first hand knowledge -we don't have that AFAIK-, to actually seeing sites in the RW). Some get discounted for one reason or another, but that has only happened with a few sources (3 IIRC).