Looking at the conversations between the MoM and Luxu, plus Luxu's conversation with Ava, I think the MoM planned for the world to end and the traitor, as part of his plan to defeat the darkness. He said to Luxu that to defeat the darkness 'we will end the world' and thereby 'discard' time and the borders of the world.
It would make you wonder who or what the traitor is. Going off Back Cover, every Foreteller seems just as lost as the other except for Luxu, but he wasn't present and accounted for during the whole debacle. So it can't be Luxu who's the traitor (obviously). And the MoM caused the discord in the first place, but I don't think he's the traitor either.
Unless one of the Fortellers were a double agent, the traitor may actually be none of them (or all of them) -- it's just whoever Darkness latched onto at the time. Maybe. Idunno. Given how things look, UX wants you to be suspicious of Ava and it kinda lines up that way, but it being Darkness itself somehow latching onto people undetected makes sense. In BC, they
did touch on how they found a Dark Chirithy snooping around the tower.
Apparently in UX, that was
our Chirithy, but it doesn't make sense because Player never really showed signs of succumbing to darkness. That Dark Chirithy probably was Darkness itself, latching onto people with the host none-the-wiser, and that's how it was able to get information about the Tower, the pods and everything else. Either that, or the traitor is actually someone we know. Hard to say.
It’s as you say, it’s most likely a War of Attrition. Honestly, what I want to understand is his motive. Why is he dealing in absolutes like Eraqus, forcing a plan that would eradicate the Darkness rather than accepting it as a coexistence with the Light? Did he suffer some sort of trauma? Was it dogmatic influence? Hypnosis?
The BoP is powerful alright, even into the current age it can bring about leftover projections on the worlds’ reconstruction, long after the first war ended and fractured the original World. In that case, the question turns from why didn’t he do it to why he did. If he didn’t write the book and mislead his apprentices, didn’t give tacit approval to the formation of unions, didn’t force Luxu to pass on No Name through the ages, none of it would’ve happened. The answer to that is that in any other worldline where the war didn’t occur, something disastrous still happens. It was a damned if you do, damned if you don’t situation. It brings about another question, was the War a necessity in comparison to the disaster that fell on the worldline where it didn’t happen?
With an eye like that though, the Master is definitely something not human, or at least, ordinarily human.
If you've been fighting a war all your life against one enemy and you've seen what it's done to your friends, loved ones and comrades... You're probably not going to want to Talk-no-Jutsu your enemies at this point, because too much has been lost, and that's something you try in the beginning -- not when you're losing, the opposition has no reason to listen to the losing party. So the MoM acting in absolutes is probably for that reason, the other options have been long since exhausted and the threat is THAT bad. It's so hard to say though because we know nothing about who the Master is or what his past is like.
In terms of preventing a war in an alternate worldline, maybe the Master was aware. He's probably thinking of how bad the war between Light and Dark is, so any other circumstances that happen in a worldline where it doesn't happen surely can be resolved with easier methods. Or, hopping worldlines is just another method of him buying time so his ultimate solution is ready in time for it to be used. Since he has the BoP, the MoM theoretically had to be aware of pretty much everything relevant up until KH3's Keyblade Graveyard when the Guardians ended up getting killed by the Heartless Tornado. After Sora used the Power of Waking, he rewrote time.
But, Xigbar doesn't seem phased by any of this, so he probably knew that everything was still going according to plan. As long as he ended up getting No Name back, it doesn't look like any major plans changed. So even though Sora overwrote time, it's possible the Master knew about that too. And if THAT'S the case... This guy is somehow seeing multiple Worldlines, because the Guardians falling should've been it. That's power of a God, or some other supernatural force, so it's clear he isn't human.
Although. It's said that Kingdom Hearts can grant untold power, and that's what Xehanort was after. If the Master of Masters was one of the first wielders and somehow was able to use the power of Kingdom Hearts... Maybe that's why he can do these kinds of things like create Keyblades on the spot for people and create Dream Eaters.
I think Darkness is a natural force of nature. Pureblood Heartless are a natural evolution of Darkness in the sense that they're primitive beings that are wild. While the Seekers of Darkness, or the Darknesses, are the more evolved species of Darkness.
You know it's not often remembered, but this would be the exact case of how Nobodies have a hierarchy. Remember how in KH2, Axel said he didn't want to be turned into a Dusk for failing to bring back Roxas? It was also said in KH2 (pretty sure) that generally people with especially strong hearts tend to leave a Nobody behind. But not every Nobody has a humanoid appearance, it's usually those with the strongest wills/hearts that end up retaining their human form. The Humanoid Nobody form seems to be the highest on the rank in terms of how the Nobody species works.
So, who's to say Darkness doesn't have an equivalent? Ansem retained his form, but he had Riku. Even before that though, he had his cloak and sentience, which almost
never happens. Sora is the only other known case, and that's because of Kairi's Princess of Heart magic (which she didn't even know how to do, it just happened to restore Sora to his human form).
Speaking of Riku, we never got an explanation for the Dark Suit and why Vanitas's suit looks exactly the same minus the color scheme. Xehanort's Vessels/Replicas in Scala ad Caelum kinda look similar to Vanitas in terms of the color scheme and the metal helmets. They can't be the replicas that Vexen made, because we see how it looks when Roxas takes over a Replica body. So where did those things even come from, and why did it become Xehanort's new armor when he has a Keyblade Armor already? I'm thinking Vanitas's appearance, along with whatever the hell those things were... Might be what this "Darkness" group looks like probably. Riku being possessed, his form turned into what we know as Dark Riku.
Vanitas is literally the darkness from Ven's heart. And seeing as how Vanitas is the only one to have control over the Unversed (he's the source of them)... Maybe the Unversed are a part of what Darkness is. Related, but different. Hmm.