Ven being possessed and used by Darkness makes the most sense for how Strelitzia's death was to play out. Especially after the information we got from Re:Mind. Leading up to this update, I was pretty much thinking it was either going to be Darkness or Gula who did the deed.
By how update ended I think this'll just lead to the scene, that Ven recalled about being chosen as a Union Leader. I think it's too early to say, if Ava is Darkness or it just took her form.
I'm kind of curious as to how Ven factors into MoM's plans it's obvious he knew that Ven would take a spot among the Union Leaders and end up in the future, seeing he witnessed it all through the gazing eye. Is this all part of his plan?
To be honest I am still on the impression that Gula is involved
somehow although I don't have a tangible theory yet on
how.
I do have a hunch that Gula and Ven might be connected somehow, perhaps another sibling pair with Gula being the older brother?
A bit farfetched at the moment I know, yet I recently re-watched Back Cover and noticed Gula's pants under his robes:

They do look a little similar to Ven's and after now writing about Ven's self-esteem issues and his diffident outlock I realized that this could come as well from a) Gula and Ven being on their own and thus Gula technically being Ven's Guardian, b) Ven often left to himself because Gula is busy with Foreteller stuff, the Lost Page and similar stuff and c) being exposed to Gula's increasingly erratic and despairing behavior including the whole "Trust only yourself"-stuff. Ventus is a 12year old kid who is shown to on the inside wanting to have friends and be social, the exact opposite of what Gula is. If Ven gets increasingly exposed to Gula's rather toxic viewpoints in the rare cases he's around he would certainly start to doubt himself and be "a loner" because that's what his big brother is too and despite himself not liking it, it is the closest role model he has.
But that goes obviously somewhat more into psychological analysis rather than any true story hints.
As for Ven's factoring into MoMs plans it might have to do with the whole "too pure" and "too kind"-attributes that are so often mentioned in regards to Ven.
Blaine and Xehanort mention it, "Darkness" does in Re: Mind and if I recall correctly both Skuld and Lauriam also comment in some UX story updates about these traits.
It also seems that despite suffering so often and so much Ven still finds it inside himself to not give up or into despair throughout any of his appearances so despite it all his heart is pretty strong and has some strong will and inner resilience.
Who knows, all the gathered Lux in Browser-Chi must have ended up somewhere and Gula did want to summon KH. Maybe that failed but in order to preserve all that Lux he transferred it into Ven's heart?
Maybe all that Lux was also used by Darkness to conceal itself within Ven and then there is of course also still the issue of Xehanort seeing Ven as good material to try and use a shortcut to get the X-blade.
See, now this is so entirely my shit that I'm going to be really disappointed if this isn't true. Though obviously that scene where MoM talked about his past suggests so many huge things that it has to go SOMEWHERE interesting like this. (Right?)
Maybe some day I should make a thread explaining how Nomura seems to borrow from the Kill the Past series, which has similar themes.
It's right now a shot into the blue without any real hints or story beats to add credence to it, but I think the main scenario could eventually expand into this or something similar.
A war is never just one single battle, that would be either just a battle or a skirmish. A war is a series of battles that can involve different armies, theatres(locations) and even times so the frame would fit even if it would be a war expanded to involve more than worlds but entire worldlines or universes.
I think
Sephiroth0812 and Spockanort ideas of how the series' plot will develop is interesting. This could pretty much be Nomura creating his own Avengers, meaning that KH3 wasn't Endgame, but rather Endgame Part I. I also have an idea that each Worldline has his own Sora and what the MoM is doing is weeding out every Worldline's Sora and make a sort of Power Rangers/Worldline Warriors team in order to get rid of darkness.
This is possibly how the MoM intends to get rid of Darkness forever: have various TRUE Guardians of Light fight the Darkness and end it forever.
Certainly one possibility, although there are several on how the actual "erasing" of Darkness could ultimately unfold. MoM states it involves lots of preparation but that too can have several different meanings.
For example, even the creation of entire new beings not by biological birth like Roxas, Xion and Naminé could be part of those preparations.
I think your interpretation is the other extreme honestly. We know MoM did awful things but Darkness is no different with killing Strelitzia and using Ven to cover it up.
Right now, even without knowing more about what else Darkness did I'd advise to be cautious about both of them. It is possible that they're both more on the awful side and I do not think that playing "who is the worst"-olympics will help anything and I doubt Sora & co would join either of them based on who of them caused less trauma and pain.
As much as I dread it, I'd say that even teaming up with
Xehanort in the future might be the lesser evil instead of choosing either the MoM or "Darkness".
That's not "no different". Killing someone is bad but hardly equivalent to destroying the whole world and manipulating countless people into killing eachother, never mind his emotional abuse of the foretellers who looked up to him.
It's not only "killing someone" but also using and abusing Ven in several ways just like Xehanort did and as ShardofTruth pointed out we don't know what else possible atrocities and other heinous actions "Darkness" may have pulled elsewhere on other characters, worlds and possibly even worldlines.
The MoM is definitely in shady territory so it might be an "evil vs evil" situation or a "this universe's gods are jerkasses"-variant so that Sora, Ven and the others need to actually defeat/vanquish
both the MoM and Darkness or these beings might simply operate on an entirely different scale of "morality" or what is actually "good and evil".
The fact that hearts and memories are immortal and with a bit of effort a being can be completely restored/reconstructed might also lower inhibitions in-universe quite some bit, regardless of the trauma and possibly lasting mental damage caused.
The latter two are frankly also all too often even ignored, belittled or even mocked in our own world often enough.