I didn't go to nearly enough seminars at Gencon, but one that made a big impression was called "History, Panic, and History Panic". It was chaired by Kenneth Hite and Jason Morningstar, two authors that have both published period-based Cthulhu scenarios. The seminar was entertaining, filled with anecdotes and interesting vignettes from the author's experiences; my second-hand notes of it are rather dry. Nonetheless, here are the excerpts - they're very topical for me, as they'll be guiding principals as I lay the groundwork for an upcoming Japan-themed D&D campaign.
The overarching advice that came out of the seminar, a guide to solving all problems that arise when running a period game, is to use common sense, don't be an ass, and have a clear understanding what everyone at the table wants from the game. If the players are there to stab monsters in the face, spending inordinate amount of time on Victorian social customs is going to bore the snot out of everyone but the GM.
You have permission to change things up, so don't feel constrained by future history or making sure events unfold like the books. Future events from actual history are a 'What If' scenario. As Ken quipped at one point, "You've stuck vampires or Great Cthulhu or magic into the setting - you've already voided the warranty".
The social arrangement goes both ways; the game master needs to understand the expectations of the players, but that one player with a PHD in Egyptology needs to understand the game master has a day job and might get some minutiae wrong. Players need to curb their pedantry for the greater good. It was also suggested the game master incorporates the expertise of knowledgeable players into enriching the game by letting them act as guides into the details of the setting for the other players.
If players don't know history, a new period can be daunting - especially because the GM has probably read a ton about the history and customs; that's why he or she thinks the setting is interesting in the first place. The past is a foreign country. Lay a trail of breadcrumbs so players can learn about the setting without an infodump; for example, recommending cinematic examples of the setting can help ease players into it. Teach about the setting through observing NPC interactions or getting direct pointers from NPCs, following the popular "show don't tell" advice. Avoid punishing the players for lack of setting mastery through the use of those helpful NPCs or NPC examples.
What about issues around social justice in the historical game setting? How should you approach racism, slavery, genocide, caste relationships, any number of awful historical institutions that engendered injustice and worse? Once again, keep in mind what the game is about, avoid dictating how players should feel or act, and ensure the players have free agency. Thus, if your game is at the height of the Roman Empire, and the players don't like slavery, you need to be willing to let the players do something about it instead of overriding choice, ala "Your character wouldn't feel that way". There could be in-game consequences for player actions, such as when they choose to let a bunch of slaves go free, but players need to be free to make their own choices for their characters.
So how would these become guiding principles for my Spirit Island game?
All you have to do is flip through other RPGs that are based in a setting like feudal Japan, and you see that 50% or more of the content is all about caste, customs, and social skills - I'm thinking of Bushido, Sengoku, or L5R, where so much of the game is posited on social interaction - saying and doing the right things, in character, in order to interact with the setting appropriately. That's great if all the players are deeply bought into the setting and are invested in getting the etiquette right. My game is going to be about D&D type stuff - encountering weird inhabitants of the spirit world, exploring misty islands, solving puzzles, kicking in doors, and getting into fights with other samurai. At higher levels, I'd expect the players to have domains and lead their own armies, too.
I'm sure my players would be interested in following some of the conventions of feudal Japan, that's all part of the secret sauce and flavor of the thing. But realistically, they're not going to read a 50 page guide to customs before making characters and wanting to throw down with some monsters. Easing the culture in via breadcrumbs, show don't tell, and using NPC "guides" will be crucial, along with taking the social side of things slow.
Placing the initial adventures on a mist-shrouded island lets the players focus on doing D&D stuff, while giving them a chance to discover the social side of the setting piecemeal as they gain levels. I think it should work fine.
Note: it seems to me this advice around historical gaming applies equally well to the Forgotten Realms and Greyhawk, where suitcases of minutiae, trivia, and endless sourcebooks and novels, have created all the same problems with setting mastery and pedantry you might encounter with historical gaming.