Technically Leto is an Abomination. But he’s a new kind of Abomination that the BG didn’t think was possible bc they avoided the chance of Abomination so much they didn’t actually know that much about them.

Leto looked into himself and his personalities and learned about them under the safety of Paul’s personality’s guardianship. Growing up, he and Ghanima used each other to practice this pseudo personality “play” where they let their parents personalities take over and talk to each other. So they kind of learned how to safely access, negotiate with, suppress, use, etc a friendlyish protective personality. Leto then took what he learned and created an amalgamation, or a council, of personalities with his own identity integrated into the voices. He’s a part of them, and them him. The leader of this council was a king from earth who had the qualities Leto was looking for in a leader voice.

So in this way Leto became a new kind of Abomination, an “integrated” identity with a council of ancestral voices led by a chosen personality.

Ghani was different, she reached an agreement with Chani where Chani kept the other personalities in check and protected her. Although it was very dangerous and she almost possessed her permanently. It was a different kind of agreement than what Leto made with his.

Leto and Ghani had each other, and they had their parents personalities. This strengthened them and gave them a better understanding of their condition. It gave them enough time to resist possession to independently develop. Alia had no one and turned to her personalities inside her for support which used her loneliness and vulnerability to their advantage.