maybe from anteiku, who had survived in anteiku raid
Actually i think the friends that Amon mentioned must be someone who have helped Amon since he was became a ghoul.
I think Arima spare Irimi and Koma
He had planned the New OEK which is Kaneki
Kaneki need his army so why Arima didn't group the army up for kaneki's sake
I don't think so that torso is belong to mutsuki. Actually, I don't know did he have done like that for all of his victims.
Torso has his chance to cut off Mutsuki's head and obsessed her torso but he didn't.
My opinion is, Maybe Torso took Mutsuki with him and escaped the Roo Island already...