Or perhaps not existing at all? Until Capcom says she's in, we'll never know.
Toshiie supporting Nobunaga. I know the Maeda clan worked with Oda in actual history but in this it makes no sense. Okay, Nagamasa's Lawful Stupid and Ieyasu's Just a Kid who to his credit openly acknowledges that Nobunaga's well Nobunaga. But Toshiie...I just don't see it.
In the case of the game, it's more nominal than anything.
In SB3, the opening movie that features Hideyoshi's seige on Odawara, Kanbe can be seen next to Magoichi. The problem is Kanbe's already in chains. Was he arrested that battle? And if so why wasn't he in Ishigakibara? Or did they pull him out for this battle?
Nohime's disappearance was never 100% explained. Oichi hints she killed Ranmaru in a fit of rage after Namagasa's death, but it was never really said what she did to Nohime. While it's entirely possible she died during the events of 2, she also could have slipped away, as historically she lived through Nobunaga's death. She also may have finally bore a child for her late husband, as her ending in 2 hints she became pregnant, and decided to live away from battle to raise him. This could be seen as surreal because she was historically unable to have a child.
In Oichi's campaign in Heroes, Oichi killed both Ranmaru and Nohime after killing Itsuki, which was the snapping point for her.
The SB3 manga confirms that Mitsuhide killed both Nohime and Ranmaru at Honnoji.
So...was it ever explained why Kotaro was in the Seto Sea? Seems like a long way from Odawara and his story didn't do anything with any information he could've gotten there.
He could've just been protecting Tsuruhime. Also, the Hojo aren't the only clan Fuma works for, there's also Hisahide.
Ok, the series shows an utter disregard for anything resembling accuracy and goes for the wildest anachronism. However, in the third game, being mostly focused around Sekigahara battle, certain "old" characters (Nagamasa, Hideyoshi, Ranmaru etc) are kept dead. What was the whole point!!? I mean, what's the point of that "accuracy" if they still keep around guys like Shingen and Kenshin (who both should be dead and buried by that time) and even decides to resurrect Nobunaga and turn Oichi into a walking Eldritch Abomination container? By the way they handled things, they could have just kept the old cast without problems, why did they suddenly cared?
The only one justified would be Lord Hideyoshi as far as I know. His death was the entire catalyst for the events of 3, causing the split into the Eastern and Western armies.