I feel like the style of historical fiction is extremely important in determining how you tell the story AND make it feel believable and immersive. I think that if you're writing IF, a totally faithful recounting of the events is completely out of the question, unless you do something like using perspective instead of real choices. A lot of historical events can be viewed differently depending on your personal opinions, so if you wrote a storygame that was an examination of events from multiple perspectives, you could keep it accurate while maintaining an illusion of choice. You could write a story on a number of controversial topics like this. Perhaps you write about the Vietnam War, but one line is the perspective of an American soldier and the other is a Vietcong soldier's point of view. The same event could be described in multiple ways, and this would be a pretty thought-provoking story. At the same time, this still isn't a real CYOA.
The most logical approach to writing historical fiction is probably the "original characters within the context of broader historical events". You can get as crazy as Inglorious Basterds or this one Russian movie that focuses on a mystical tank hunt. Both of these movies are ridiculous, but they still feel fairly real. You could also do something like Jojo Rabbit, just more serious. One child in Nazi Germany can have any number of reactions to what's going on around them, and could be a really neat setting for a storygame! Overall, there's a ton of options if you follow this format, and I think this is the most broadly used format of writing historical fiction for this reason. You are also able to fudge more without going into unreasonable territory.
The "what if?" approach can also lead to some interesting stories, although I think the best way to go about this one would be with more subtle changes that can have a butterfly effect, or you can just focus on a real story, but with fictional options added in. You mention the story of Matilda Friend, which could easily become a CYOA where you play as her and are trying desperately to survive. Maybe you save others as well, or maybe you just die. I could write about my great grandma, whose first husband died of TB, whose son disappeared mysteriously, whose second husband was fatally wounded by a German bomb, whose sister was killed by a land mine. I could add branching to every part of her life and result with the potential for a story that is a mix-and-match of these various real events with fictional events.
Folk tales also provide excellent material, because they are unverifiable and give you a lot of creative freedom, so that's another route one could go down.
There are a ton of ideas for historical fiction that I've come up with just in the ten minutes or so that I've been writing this, and it would be awesome to see more of it on the site, but I don't think I'll attempt it for now.
Good thread, not sure I can add much to it though.
Speaking of incorporating history into fiction, I thought the prelude mentioned in this video (around 8 minutes) was a cool concept, and tried having a similarly styled opening for a sci-fi story (not that it really worked out).
Other than that specific example, generally being inspired is a good way to get ideas, even if some of the coolness factor may be lost translating something that happened into a fake version. But just learning stuff is a good way to broaden one's views and be capable of having more interesting ideas (I'd say).
Regarding useful resources, watching videos is easier than reading (for some people) and not doing the research yourself is more convenient, so OverlySarcasticProductions is pretty neat (yes, my previous link was to a video of theirs). There's lots of history related stuff on their channel. Could always look into the events they mention yourself as well, but I imagine they're not just making shit up.
Oh, they also have some writing related videos, which (given the site) is relevant! They've also got a good deal of mythology videos.
I think that the "research" for a good historical fiction story is something that had to have been done before the thought of writing the story even crosses your mind. Absorbing the amount of "minutia," as you say, that you needed to have taken in in order to write a good historical fiction story consumes a great deal of time. When I wrote Warlords, I had already read a lifetime's worth of medieval Chinese books, military treatises, war video games, and Wikipedia entries and history textbooks on the setting and subject. Additionally, I've always liked historical fiction stories- the first book that I bought with my own money was The Legend of Tarik.
As I get older, the more I see that a lot of "history" is fiction anyway. I'm no great writer, but my advice would be to focus more on the fiction, and less on the historical. People read historical fiction for the drama involving familiar characters and settings, not a history lesson, imo. As long as the setting feels realistic enough, it should be an adequate backdrop for an engrossing narrative. History should inspire, rather than constrain.