I'm a fumbling a bit with the details revealed in this chapter but I'll try to piece it together to come up with a somewhat coherent explanation for it. wish me luck. D:
It seems that there are 2 Reikis: the original ceremonial blade and a facsimile created--more like a sword she imbibed with the properties of the Amaterasu Gate--by Senhime 400 years ago. Iyo, the spirit of the true ceremonial blade, was already channeling her ki to "create a path" between her and Susanoo for Aya and co. the other reiki, the faux reiki, wielded by Aya was used to neutralize Souhaku's ki in the world of Susanoo's mind, leading to the loosening of Souhaku's grip on the body of Souichiro. That was the reason Souichiro reverted to his normal appearance and was able to punch through the faux reiki through his old man.
that last page was a bit too cheesy for my tastes. it would have been fine--fitting is more appropriate since their ancestors fought against Souhaku's schemes ages ago--if Souichiro, Aya and Makiko were the only ones ending Souhaku's life but Bob, Asshat and Kurei were included? LAME.
I still hope, a tiny sliver of optimisim, that Shyoujou and Kabane are still alive and kicking. I know it might be a longshot but I really liked those two and it would be such a waste if their role was that shortlived.