Also why did the rest of the gang just assume that the other two have been killed since ch...

Maybe they're just so fucked in the head that's where their minds went to. "Well I'd do this so obviously someone else would!" Kinda mentality

I think Okuya (or whatever the boss' name is) said that in the passing and basically implanted that thought in them :>
he's a lil extreme bastard, I don't understand his brain

Yeah I hope that the author makes his end satisfying for the readers