I liked these personalities. ending was bit unsatisfying but better than I feared. I was hoping that at the end they would be able to coexist. Also Zero kinds of drifted away. Do they say merging with the trio, but was she really pure anger to grab? I think she was more.
In the end, it was all just a theory characters agreed on. I have one on my own:
Zero wasn't original, neither Mokoto later appearance. Zero was like they say patient zero, the first personality split emerging. Zero with all the bad memories was pushed away and Mokoto emerged as a suppressor of Zero because she had no power to do anything with her rage at that time (child, too weak). Sometime later when Mokoto couldn't keep it up I think that original (5-year-old kid) simply ceased to exist anymore further splitting into 3 other personalities. They all are just a part of original, which no longer exist.
That pyramid scheme (pun intended) is just that, scheme, no actual value in it, just a scam. No one was on top and bottom.
The way I see it, Zero was the lashing out personality, angry at the world and herself for her childish outburst. Makoto later the one keeping it together without breaking, pretending everything is fine and Hibiki, Fujiko and Mikiri the ones to keep up with the fathers. Altogether, I think that it's hell messed up family and I get it why mother of Mokoto wanted to run away from that with her child. Mokoto needed help as a traumatized child then but got into training various martial arts instead. They keep her body safe but shattered her mind. What Hideo picked up at the end is just a pieces glued together from a broken person. Never to be same again.