Now that is some high-level plot-convinience. I don´t even know where to start. By Yumi figuring out that Akane and MC had sex without any hint? Yumi having super strenght all of a sudden? MC falling asleep at a moment where his adrenaline should have been trough the roof? Or by everyone suddenly disappearing? And what was this random naked dancing scene?
What a load of crap, even for a manga like this.
Reiji was blushing and averting eye contact when Yumi was staring at him, he ain't hard to read.
Yumi got huge adrenaline rush and being driven insane by being raped recently, can push body to go beyond usual activity.
It's been two days already, he didn't eat, he had sex with Akane and after adrenaline pumped escape, him dosing off from exhaustion isn't surprising.
There's only six people left, pedophile is stalking the girl all the time, Akane plan was to set Yumi on Reiji so probably heard the commotion and decided to not interfere. Yokitoshi came looking for them elsewhere probably.
Yumi went fully insane, think she tried to relive moments of her love times with Yukina senpai, she was a gymnast. It's crazy, she went crazy.