Akihito is just like any full-blooded youmu, and taking out his eyes would kill himEpisode 1 introduces Akihito's immortality as if it's unique, but in episode 2, we discover that youmu apparently can regenerate indefinitely unless their eyes are destroyed. The same rules likely apply to Akihito.
The scar on Hiroomi's back is from Akihito.It happened during some previous instance of Akihito's youmu side taking over.
Theories on Akihito's dad.There's gonna be some sort of huge revelation involving the dude. Place your bets!
The old man in the rainbow cave is a youmu possessing the Nase family.Who is the mysterious subterranean elder giving orders to the Nase children?
Akihito is not immortal because Beyond the Boundary is possessing him. It is possessing him because he is immortal.Despite what was said in Ep 11, he did not take any sort of damage that would test him after that. His coma was not because he was no longer immortal, having Beyond the Boundary ripped out would have killed him otherwise. The trauma (plus the Calm) meant it took him that long to recover.
The Kuriyama Mirai we saw inside the Kyoukai no Kanata was not the real Kuriyama Mirai, but an image of her created by the real one.Recall that before she actually fought dreamshade!Akihito that she took her glasses off. Recall then that when you see her again inside the Kyoukai no Kanata that she's wearing glasses, the glasses she saw inside Akihito's Megane magazine and the ones he wanted to get her on her birthday. When we see her breaking apart, it's because she's no longer holding the Kyoukai no Kanata in her body and thus her image created by both her and the Kyoukai no Kanata are breaking apart. The reason we see her again at the end of episode 12 without glasses is because she has recovered on her own.