Why I think that Mana was in fact truly an Akuma and not some form of false memory
Dgm is filled with mysteries and paradoxes, and the relationship between Mana and the Earl of Millennium is one of those. On the surface, it is contradictory that Mana could have been turned into an Akuma when he is later revealed to be the Earl. However, the narrative provides subtle hints that may reconcile this paradox. (Bear with me guys)
1. Dual Identities and the Split of the Earl
The Earl of Millennium isn't a singular, static entity, this we already know. Over 52 or so years ago, Adam split himself into two infants in front of Katerina who would adopt and name the infants 'Mana' and 'Nea', raising them as twins. Mana and Nea were both integral parts of the Noah family, and there's this suggestion that Mana might have been a vessel of Adam's memories and thus was the one who became the Earl of Millennium while Nea could not, because he likely lacked something. When Mana lived as Allen’s foster father, he have suppressed/lost his memory of being the Earl due to his grief-stricken mind and deteriorating mental state. This allowed him to exist as a separate individual while being consciously aware of the Earl's existence.
2. The Akuma Creation Mechanism
The Akuma creation process involves a soul being bound to a body after someone calls it back from death. If Mana’s "human side" died and was mourned by Allen, the Akuma process could have targeted only that fragment of Mana’s identity. If the Earl of Millennium that appeared in front of Allen is indeed Mana at that time yet still present for when "Mana" was turned into an Akuma, there's a possibility that the Earl at that time was simply the Golem Suit, Mana himself but his body usurped or, as mentioned above, because the Akuma is Mana's human side.
3. Mana’s actions as an Akuma
The corrupted Mana attacks Allen, since his human persona was overridden by the Akuma’s violent instincts. However, his plea for Allen to "keep walking forward" does suggest that Mana's original personality wasn’t entirely lost. One is free to think of Allen's cursed left eye, which gives him the ability to see the souls of the Akuma, is a punishment but i saw a theory that the eye is the Earl's eye(?? I forgot the exact wording or speculation/theory but I got some of it from: @poppystheories ) although in a sense that Mana's curse serves to warn Allen of the presence of Akuma, of the tortured souls and also of the fact that the Earl's "arms and legs" (or was it "eyes and ears"?) are near. And we know from Alma that Akuma are offshoots of the Earl, which is how he knew that Allen was a Noah.
4. Foreshadowing and Symbolism
Hoshino frequently employs themes of duality and identity in DGM. Mana’s transformation into an Akuma while also being the Earl symbolizes the conflict between the Noah and humanity, as well as the tragic cycle of grief and that drives the story. This paradox could serve as a microcosm of the series’ central conflict, also the duality of having humanity and an inhumane identity can also be seen in this.
In Short: Mana’s transformation into an Akuma despite being the Earl of Millennium is because Allen called for Mana Walker rather than his Noah identity as the Earl of Millennium. Thus why the Akuma Mana could exist at the same time as the Earl's, his curse also have warned and er- "protected"? Allen more than once. It's also undermining if the Akuma summoned by Allen turned out to be some rando soul that the Earl used, since it doesn't make sense. The only plausible reason my mind could come up with is that the Akuma Mana summoned was the human aspect known and remembered by Allen.
