I take what happened is that Keiichi did take what Rika said about Satoko to heart and spoke to others about it, and the memory leaks allowed them to take her seriously.
He might have taken one for the team by letting his guard down while being alone with Satoko, while making sure others, or maybe just Rena, to keep an eye on Rika no matter what if he does go down. Meanwhile, Mion and Shion captured Keiichi once he L5'ed, and no one got killed. Instead they might have just subdued him and got Irie to check on him, maybe lowering his L5 if possible.
And ofc, Satoko got too cocky. She only focused too much on Rika to notice everyone else's movements. She already questioned Rena, but since the very start, her focus are always on Rika and Rika alone. She never thought about the other possibilities and thought her methods are 100% flawless.
She's either going down this Fragment, or everyone will take her down in the future Fragments the longer she tries to loop.