no, you just need to have seen the 3 main endings. There's no way you can get them all in one go.
Dong Zhuo and Jia Xu he hid as his personal servants to prevent the coalition from tearing them apart, so to speak. Lu Bu surrendered after being promised food and not to destroy her home.
Yuan Shao and co. never join you, but you beat them up and take over the northern part.
Cao Cao falls under a spell by Gan Ji(one of the two sorcerors) and moves her army against you. Beatings all around and finally her underlings pick up that something is wrong, go to you for help. As gratitude, she surrenders.
For Wu, there's a split in power as Zhou Yu tries to worsen relations and Sun Quan wants to patch up. Finally, Zhou Yu launches a coup. During the process, Sun Shang Xiang decides to try attacking and fails, getting caught, Sun Quan, Gan Ning, and Lu Xun join you in a request to retrieve the Wu lands. Scenario results in Zhou Yu sitting alone in the burning palace.
Cue final battle. The two sorcerors finally decide that it's time to close the book on this "Gaiden" world. (An alternate telling of the true version) The team marches as the Wu and Wei stop the bulk of the enemy army, letting your main group pass. Inside, Kazutou touches the mirror and thinks about everyone.
About the same time, Yuan Shao and her two underlings arrive at what appears to be a deserted island and overload a matching mirror.
Results: everyone but the sorcerors and Hua Xiong are warped. dead people like Gong Sun Zan and Zhou Yu are revived as well.
So...timeline wise, I'd say this would stretch to...roughly the the battle in Jing Province, given how much time passes.
Just for reference, you have around 14 intermissions to spend 2 major character events and 3 minor characters events. And no double ups. Main characters are 12-13 intermissions worth of time. the minor characters are variable(2-9).
For the harem ending, do you have to like what, do all the girls or something () ?
In fact, how did he win over his enemies in the first place? I got a feeling the game time doesn't stretch that long And that's not to mention the other three girls that are already on his team... How do they fit in this orgy?
Each main ending(Guan Yu, Zhang Fei, Zhuge Liang) takes him back to his time alone with the girl. The harem end involves Yuan Shao's trio, finding an extra mirror roughly the same time as the final battle and causing an overload while Kazutou was thinking about everyone he's met to cause nearly EVERYONE to make the jump.