Thank you this helped understand a bit more
About 6 this also explains why they tortured potential exorcist in the begging (in the case of Lunali) in order to rise their compatibility with innocence
Yeah, innocence probably responded to the strong energy of her DNA/Soul as Lavi put it that was due to the torture.
The last point: Allen is shown deaging when he’s being deconstructed by Apocrophos, so it has more to do with that than merging with Lavi.,
I'm doing this from memories and injecting my own conjectures along the way. Just wanted to bring up some major lore details we have been given throughout the years.
1. The Noah are a clan of 13 individuals. 12 of whom have continuously reincarnated over the course of 7,000 years according to wiki. The Millenium Earl or Adam is the only individual to have retained his original form at least up until he split into two, Mana and Nea.
2. The Noah are the original Humans of the D Grayman world. Every single human being is descended from them and carry the potential Noah Gene which enables them to inherit the memories of one of the original 13. (My own theory: The Noahs may hate the current Humanity because they have forgotten their hatred for innocence, Envy their ability to die, or have chosen to join innocence's side).
3. Innocence Destroyed the Original World that the Noah came from. This is why they hold a deep hatred towards innocence and seek it's destruction. Well Road also said they were the "Apostles of the True God" whatever that means. Based on a small snippet during Road's speech, it seems their world was far more tecnoologically advanced (skyscraper and tall buildings). So either this is a different world altogether or the world reset due to innocence.
4. Prior to the start of the story, Nea did something unforgiveable to the other Noahs. He then was forced to flee. Somewhere along the line, he met Allen, a soldier. Allen became Nea's host.
5. Lavi, the original bookman heir, was friends with Nea and found Allen. The two were journeying together for a certain purpose. Eventually, they met Apocryphos who wanted to destroy Allen/Nea.
6. Original Lavi protected Allen by sacrificing his existence. Here, we learn that Innocence is like a parasite. It leeches energy off it's host. Specifically the newest chapter seems to hint that innocence particularly likes negative energy. This is why all the exorcists are so miserable/have miserable pasts. Innocence seeks out miserable/despairing people.
7. Lavi merges with Allen. However the energy released by this merging of Lavi and Allen is too much for the Heart Innocence to absorb and for 7 years, Lavi, Nea, Allen, and the innocence are stuck. Perhaps this is what caused Allen to regress in age. His body has literally been recreated.