Up to this point, we've been lead to believe certain people are guilty until it turns out that they aren't (or at least have a strong case for why they wouldn't be). First it was Lauriam, then it was Brain. Just like what others have mentioned already, not being chosen may not necessarily mean they're also Strelitzia's killer. Ven definitely seems to have a lot of darkness within him if Vanitas is anything to go by, but we don't know if he's always had that from the beginning OR if that darkness developed after what happened in the Age of Fairy Tales/Meeting Master Xehanort. So far, he's acted along the same lines as how we know Ven acts currently.
Plus, something to keep in mind... If the Master of Masters knows what the future holds, he absolutely would
know about Ven, considering just how big his role is. Taught by Master Xehanort personally, ends up with Master Eraqus, Terra and Aqua, and then becomes one of the Guardians of Light in KH3. If anyone should have the Book of Prophecies, it's actually him. He's the most important piece in all of this next to Sora and Xehanort.
When it comes to Ava being Darkness? I think there's a good case for it, but I don't think I want to make that accusation just yet. Luxu says that "Ava had her own mission, and she carried it out." Now, we thought that it was her selecting enough capable Keyblade wielders and forming the Dandelions. But now that we know that she acted under her own agency when giving Brain the Book of Prophecies, what IS her mission? Given what she's said to Brain, she wants to change destiny, even if it means going against her own Master. But at what cost? Luxu didn't seem all that surprised that she wasn't summoned back, so I wonder if he knows what her plans were. Maybe he has an idea but nothing concrete, a hunch of some sort.
"Observations, Excerpt 1— I have seen it through; the Keyblade War unfolded exactly as written on the Lost Page. Now, the Keyblade the Master entrusted to me must be bequeathed to another. Five Union leaders have been chosen from the surviving Dandelions. I will pass the Keyblade to one of them, and then continue watching the future unfold. Yet it seems that someone has pulled the old switcheroo. One of the Five is an imposter, someone the Master did not choose. They represent a virus in the program he so carefully wrote. The virus has begun a strange undertaking: a reckless plot to allow the Five to escape into another worldline. Surely such a thing can't be possible? We're talking about the same trick that allowed the Dandelions to transfer to other worldlines after the Keyblade War. But these children are no Masters. They haven't the means...unless, of course, a certain lady of magic summoned here from the future knows more than I do. The whole Union leader thing was supposed to be by the books. Are these new events just another phase in the Master's grand plan? —Unknown
And... "Even on a worldline with no Keyblade War, peace is but a dream. In the absence of us and our Master, a "darkness" arrived—one that shall surely lead the World to yet another demise."
It makes you wonder. Darkness is the one that informed Maleficent of how to jump back to her original timeline, especially noting that going to the Daybreak Town tower will allow her to return to the real world, and the method for returning to her time is deep in the tower's real-world counterpart. The tower (if memory serves) is forbidden for everyone except for the Foretellers/Union Leaders and the Master of Masters, so how would Darkness know about this? This is secret information only a handful of people should be privy to.
Also... Ava is the one who chose Brain to have the Book of Prophecies, but with the latest update... He lets Ephemer see the list of the Chosen Five. Meaning, Brain and Ephemer were definitely chosen by the Master of Masters (unless Brain is luring a trap and made a fake list, but then, why?). Brain was looking over the BoP and then gets his Chirithy to summon Ven and Skuld. Right after that, he's talking with Ephemer. Everything currently going on is a direct result of Ava interfering. Plus... None of the Five would know who'd be chosen before everything happened, so Strelitzia's killer would have to be someone who knows she'd receive a rulebook.
Strelitzia was chosen and then was killed the next day after she realized Player isn't in the Dandelions and wanted to tell them the truth of the fate of the world. The Master of Masters told Ava that the Dandelions won't need to remember the tragedy that is the Keyblade War, so it's implied that a Union Leader talking to ANYONE about anything before the appointed time is a huge no-no. So what happened? My guess is, she was silenced. And if this is the case, then there's only one person who could've done it.
Now, switching gears and going back to this event:
Ephemer was surprised that Skuld was also chosen, but Skuld assumed Ephemer would be chosen. It might be me looking into this too much, but it's an interesting detail to note. Now Ephemer is asked about the person "who isn't meant to be a leader originally". Food for thought.
Another EXTREMELY important detail is the exchange between Ava and MoM: "Yeah, it'll be dangerous if anyone but little red circle there sees it. So make sure you only give it to them, and in secret." Right after that, Ven is revealed. However, we know that Ava gave Brain the BoP and now we ALSO know that he wasn't supposed to have it. So who was? I think it was Ven. Like I said above, it just makes sense if it turned out to be him, when you think about the role he's played in the series up to now.
All of this is a guess, but one thing is for sure -- next month's update is going to be crazy.