I’ll go over my understanding of it and why I think it makes the most sense that way.

Nothing really changes from what happened with Saint before and how we saved him. The thing in question during Echoes was whether or not “real” Saint was saved, or some other version of him. It all started when the Conductor yoked him and controlled him for a bit. When she did that, she told him that he’s just a copy and he started believing her because he was suddenly experiencing some differing memories.

I say suddenly because this never happened before, including in Echoes; the first radio message in Echoes was Saint and Osiris reminiscing about their past together and recalling the same event without any indication that their memories are in any way different. This event is also shown in one of the lore tabs from Echoes.

Radio message:

Saint-14: Osiris. Do you remember when we knew?
Osiris: Knew what?
Saint-14: That we were meant to be together. There was one moment… though, it took us time to get to it. Oh, but our Hunter Vanguard, so smart, Tallulah… she knew before we did. All our little squabbles and bickering. She saw it first.
Osiris: it, uh — ah, well, I was — obtuse. Stubborn. I couldn’t recognize my own emotions. Then, Tallulah told me to… “Be serious”.
Saint-14: Haha! I remember. So yes, she told me later. Was good laugh.

Lore tab:

Tallulah raised her hand, a single finger extended. “You notice every detail about him,” she said.
She raised a second finger. “He distracts you,” she continued.
A third finger raised. “Yet you can’t make yourself avoid or ignore him.”
She rolled her wrist, letting the thought continue unsaid. Osiris stared at her.
“And?” he said, crossing his arms over his chest. The Hunter laughed.
“Be serious.”
Osiris blinked. “Excuse me?”
“I need to make it clearer? Okay. You two should—”

They are clearly talking about the same thing, from before the Infinite Forest, before they were even officially together, so this event is basically set in stone in our timeline, with our Osiris and our Saint. It means that Saint’s memories were perfectly fine and aligned with Osiris’ (and everyone else’s, because Saint having different memories from everyone around him would’ve been pretty noticeable to people prior to Echoes).

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