I get the feeling that Arima will not be able to slay Shachi at this point in the plot. Yomo has Rize in his safe custody yet Shachi has not been able to break Eto out of her predicament. I am guessing :re will intervene and pull Shachi out of harms way, which will further amuse Arima as very few ghouls have survived multiple encounters with him (the only known exceptions being Eto and her father).
In terms of parallel plots, it was at this time that CCG and Anteiku raided Aogiri Tree, and the ghouls counter-attacked CCG. Note how Urie mentions simultaneous offence and defence. Note also how the Kaneki-Jason torture is occurring at the same time as the Torso-Mutsuki debauchery.
There will definitely be a simultaneous clash. One at CCG and the other on Rue Island.
The other interesting point I noticed from the first series is that in relation to the Arata prototypes used to combat Kuzen in that brief standoff is, Kuzen said that the Aratas did not wish to conform to the CCG investigators' wills. Amon is in possession of a third Arata prototype. Will he able to call forth its powers to the fullest? As the Corpse Stealer, Arata was a pacifist and only ate to survive and feed Touka and Ayato; as the cannibal floppy, Amon has being doing this since he was ghoulefied. I can't wait for him to appear this arc, he is probably one of the few players left who can stop Kaneki - on this point, at V14, Arima said "SSS" when he confronted Kaneki. Since he has been absorbing Arima's techniques, eating Eto's lower half and restoring his memories, Kaneki has probably surpassed SSS strength by now. Only Eto, Arima or Amon can stop him at this point. Or V, but they seem reluctant to directly intervene other than by using their CCG agents.