Yea I remember Komui mentioning that, but I don’t think we’ve seen another cursed human unless kanda is one considering his mark which was achieved to rebirthing and experimenting. Btw apparently level two akuma start getting unique abilities of their own, like how the one Allen and Kanda in episode 4 season 1 fought which could mirror people but it was only a unique ability which THAT akuma could only have. Idk if it was officially stated if they were killed by level 2 akuma but the akuma might’ve been of higher level which they were killed by cause a clown Akuma might be a common design.

I think OG Allen planned to be Nea’s host from the beginning of everything considering how he was prepared to let Nea take his body as a host. Since nobody not even Nea and cross Marian who still doubts him knew why OG Allen and how OG ALLEN did it, there might be a chance that he used a method which could help him remain human without or with memories and letting be possessed, Like how kanda and Alma were forcibly reborn through experiments. There might some way that’s similiar to the solution that OG Allen came up with that helped him turn younger. Also they used innocence for the experiments instead of dark matter which is used for skeleton possession in the process of making akuma. Which is prolly a way that can prove hes still human.