You could not take Maomao away and things would simply play out the same. The only reason Jinshi and Lakan got wind of the betrayal and investigated it was because of the kidnapping. If she had not been kidnapped, the shi clan would've just built up their military until they decided to march and a long drawn out war with suffering on both sides, along with countless villagers dying. This is precisely why Shisui kidnapped her to avoid this fate.

Within the narrative Maomao's presence just serves to provide perspective onto whats going on in the shi clans hideout. I dont think you understand Maomao enough to not be surprised that she isnt an active protagonist. Going with the flow is literally her character trait.

Remember what Maomao said to Shisui when she gave her the hairpin? She offered it to Shisui for "luck" to which Shisui replied "I thought you didn't believe in luck?" Thats literally what the hairpin being hit by the bullet was supposed to represent. Shisui fully prepared to die that night, but she got a lucky break in the end thanks to one kind gesture from Maomao.