What if the killer is Gula, who used the same illusion powers Ava used to make himself look like Ventus? Strelitzia was killed in Gula's building after all. He took the book, and choose the likeness of a relatively lesser known wielder to fit in.
I initially liked this idea, but the more i think about it, it has a few major issues.
Where is the real ventus? How/when does he get swapped for the Gula version of Ventus and why did Gula get summoned back to the future with the other foretellers if they are gone, but Gula isn't?
To me this set of scenes all be convinced me that Ava is darkness. They have admitted to reading the book of prophecies. To me, only 7 characters have real knowledge of the book at this point. The Mom, 5 foretellers, and Brain. Stealing Strelitzia's spot/rule book didn't get this person the Book of Prophecies.
In any good criminal case, you need to prove three things to convict a person, Motive, Means and Opportunity.
Let's tackle Opportunity first
1. For 4 of the new leaders, we don't know anything about their whereabouts at the time. The only one we have any idea of is Skuld. She was with Player until right before. We know that Skuld was nearby the house Strelitzia was murdered in at the time of her death.
Means
1. All the characters have the ability to kill her. We are dealing with powerful key blade wielders after all.
2. However, even though everyone has the power to kill her, who knows that killing her would provide them an opportunity to infiltrate the leaders ranks?
Motive
What motives exist for killing/replacing Strelitzia or any leader for that matter? Below I have listed out the ones I can think of, no matter how unlikely.
1. Becoming a union leader will guarantee them access to something/someone that they otherwise would not have. This could be the BoP, this could be the Ark, this could be the foretellers room / clock tower to search for secrets (but one would need to know these things exist, and how would anyone know about these unless told be a foreteller? Perhaps one discovered something upon exploring the tower previously? We know Skuld and Ephemer have been inside...)
2. Strelitzia was threatening someone/something and needed to be stopped. This feels incredibly unlikely, and we have no basis for any indication that this is the case. Perhaps this person read the BoP and determined the actions Strelitzia would take would undoubtedly ruin the chances for future success. The only possible person here would be Brain, if he had killed her and replaced her with another leader, then he wouldn't even open his mouth, let alone show Ephemer, so I think this is out.
3. Along the same lines, her killing could have been 100% random and had nothing to do with anything. Then upon taking her belongings, the killer stumbles across the rule book and decides to take her place. This is obviously incredibly unlikely and ultimately would have no real positive reason (story wise) to happen.
4. To plant another person in her place. Long thought by many (myself included) to have been Ava quickly replacing Strelitzia with another potentially more capable leader. Well what if another fore-teller had done so? Eliminated a leader to identify a replacement? The problem here is, how would they know of Ava's role, and who was chosen? How would they know who to replace them with? If they did do this, how then would they communicate to the replacement leader? If the replacement leader is unaware of Strelitzia's murder, then why would they hide that they were given their role by another foreteller? Why not speak up and say "Gula gave me the role and book?"
5. To change the fate that had been designed. This has been speculated for a long time as well. It's the only motive for the swap, that involves the new leader having no idea it happened. This person then doesn't need to have been given loads of information no one else had (about the ark, the book, how to interact and change the future etc. and a reason for existing as a leader, since it doesn't appear anyone is trying to undermine things and prevent an escape back to reality.
In the end, the motive that makes the most sense to me, is still #5, the intent to change fate without knowing how, which means it would have to be Ava. Yes this cutscene suggests that it wasn't her. But Nomura loves his red herrings, and all this was to pull suspicion off Ava was a statement by brain that says "I don't think the change in Union Leaders was Lady Ava's doing" this could easily end up that Brain was simply wrong.
Looking at the evidence,
Who knew Stelitzia was a union leader? the MoM, Brain (potentially if he already had the BoP at this time, but again as described above, why show the list ever if he had killed her? No one would ever find out), and Ava. She has already shown a desire to change fate by giving the book to Brain. If she wanted to really change fate, then changing the leaders was her best chance. I believe she was questioning things before the conversation with Luxu, as seen in the conversation when Brain was made a leader. After that conversation with Luxu, I believe her whole mind snapped, (with Luxu claiming she was the traitor for making Brain the man with the Book of Prophecies, which would then explain why Luxu called him the virus, even though he isn't the traitor/unexpected leader), and she lost it. She then determined to unravel the MoM's plans, took out the first leader she found, desperate to instill someone other than Strelitzia. My guess is Ven makes the most sense as he was just a random she grabbed off the street, and it would explain why a Meh wielder who isn't nearly as powerful or as successful at gathering Lux is appointed leader. It also explains why Ava would then go on to become Darkness and be so desperate to help Maleficent and the Dandelions escape back to the real world. She was desperate to undo everything she had set in motion and stop the MoM's plan.
the TLDR version of my ideas is this:
Ava is still the bad guy, she did it all to change fate without knowing what would happen, possibly explaining Darkness' statement about having read most of the BoP (reasoning being, I changed things, so I know most of what's coming, but not all)
I also agree with whomever stating that Ven being the killer will irrevocably damage the character in ways that make him unredeemable, and I just can't see Disney allowing that to take place.