My reasoning is that the conversation with Luxu (which happens after Strelitzia is given the book) changes something in Ava. She is compelled to change the future. Or rolling with SuperSaiyan's theory, Strelitzia is about to blab and needs to be silenced. Either theory works well.
As you pointed out, Darkness being from the future and traveling back is highly unlikely given the phrasing of things. Ava wants to get everyone out of the Data world which would change the future. Luxu states in the secret reports that a "certain lady of magic" can help them do that. That is why Ava needs Maleficent and would want to help her.
This is the video that turned me on to the idea. If you want some more reasoning to why Ava could be darkness, I'd check it out. All credit to the Secret Reports for creating it.
Killing Strelitzia to silence her is still a bit too much. Ava is a Foreteller, the easiest to talk to, even from other Unions, she is respected and loved. All she had to do was to ask Strelitzia to stay silent, or at worst just knock her out until the war starts. If she wanted to change the future by replacing Strelitzia, she could simply ask back the rulebook. Killing when you have easier methods at your disposal just doesn't add up, especially for kindhearted Ava.
The video brings up some intresting points, but I'm still not entirely sold. The "venture forth in secrecy" can simply mean her changing who gets the BoP. And still, Ava's goal would be to rewrite the future and help the Dandelions. Why kill one of them, why help Maleficient? Also I don't belive the book they found in the room from which they made the spirits is the BoP, just a random book.
As for the "certain lady of magic" I belive Luxu actually doesn't know about Darkness, and the information he refers to doesn't come from Maleficient, but Darkness (info on the ark, the thing that possibly takes Lauriam, Ven, Elrena and some to the future).
What I belive:
- Ava is innocent, other than giving Brain the BoP
- Darkness is not from the future
- Luxu isn't Darkness and/or the killer
- the killer is not the replacement
- the replacement is either Lauriam, or Ven/Skuld/Ephemer
- if it's Ven/Skuld/Ephemer, the killer was posing as Ava (I exclude Lauriam because I don't remember him ever mentioning Ava choosing them, like the others, and he might simply got the book from someone else, not knowing it's weird)
- Darkness is a Foreteller (due to calling MoM simply Master; taken from the video)
- Darkness is the killer
- since the replacement is innocent, the killer belives this change is enough to cause a big enough change in the future, even though the replacement doesn't know they are special
- Gula is Darkness/killer
- his goal is bringing MoM back by simply messing with the future, not caring about the kids
- Ven is the replacement