Notes

My headcanon is that the looming threat Mituna saves his friends with is being sucked with Lord English into the Black Hole that forms from The Green Sun in Act 7. That in the end, one of each ghosts survive (alpha and beta trolls) and are about to get sucked into the Hole with Lord English, then Mituna flies between them and the Hole, fires his accumulated psiioniic powers and opens the gate into the White Universe (the one the Kids and the surviving Trolls end up with after entering the White Home). That the sacrifice Mituna makes is sending all his psiioniic powers into the future, to meet inside the final, last surviving Mituna's mind and grant him enough power to open the gateway.

Explanation patched from my e-mails with a friend:

He fires a powerful lazer of Red and Blue light towards the Green Black Hole. Do you know what happens when you shine a perfectly red and a perfectly blue light at a perfectly green spot? White. You get white. The White Hole sucks everyone in.

My interpretation? Well, timelines and shit, but all in all, the White represents the universe the kids and trolls created together. Hence they all end up there in the end. And why wouldn't Mituna save his sanity until the end, you might ask? Well... Let's count... It cost a life of one experienced, thousand years old PsIIonIIc to get one ship to fly with the speed of light. One Mituna couldn't save all his friends from a gravitational pull of the whole paradox space (which is multiple universes) and it took all the Mitunas in Paradox Space, all their combined psionic powers to achieve that, and all those were transferred to this one final Mituna.

- Mituna is the Heir of Doom; So one who becomes... The way John became Free (Breath) and Equius became unimportant working in the background (Void). Notice anything? They don't become the aspect itself, they become the adjective of the object; So Mituna doesn't become doom... he becomes DOOMED

- Assuming his psiioniic powers free everyone from the eternity (served cold) in the black hole with LE; How do the Captor powers work? They come from the eyes. And the only person that Mituna can't ever look at? Himself. He can't save himself, he's the only one to be doomed (look above xD)

- There's not a version of him that remained sane [or at least I didn't find a reference to it in my research]. Of a guy that could foresee danger and steer his crew to avoid it. That means the danger was unavoidable at that point! So it must have been something that was bound to happen later! And I think that's why Dreambubbles couldn't heal him, because it was the damage he was supposed to suffer through to empower the final!Mituna
Still don't know about Kurloz being there, and how it fits into the theory, the o- I know. I know, nevermind.

Kurloz has a nightmare, right? When he blows Meulin's ears up. The Vast Honk. It's (headcanon) about Meulin dying. Or LE being defeated. Or something. He bites his tongue off. Maybe because he blames himself for Meulin, or maybe to keep the secret from everyone else. Because he worships LE and doesn't want others to know how to defeat him. And eventually, Mituna develops a bad feeling about Kurloz, and his silence. Because Doom and all, makes sense? So he confronts him. And maybe Kurloz breaks, maybe Latula helps Mituna read Makara's mind, maybe Kurloz confesses to Mituna as a moirail. In every timeline it plays differently. Either way, Mituna knows. And being the Doom player, he sees the last battle, the black hole, the eternity inside the black hole... and realizes what needs to be done.

I think Kurloz, being a Prince, and being a Prince of Rage, helps Mituna there. Why?

In my head a Prince is the one that has his Aspect mess with him, hurt him or abandon him. In that he governs it with others, but he himself lacks, at least in a literal aspect. And look: Eridan loses all hope, and Dirk doesn't form a romantic relationship with anyone other than the hella failed one with Jake. He's the only one that doesn't get hyper in trickster mode, because his heart is so empty. Then it means, that Kurloz lacks rage. He worships LE, he's open about his goals of killing his friends for LE (there's quotes on the wiki), but I can't help but feel he's the happy mirthful messiah, the :o) to Gamzee's :o(. Why? He's calm, he doesn't kill anyone, he treats people nicely (his actions are way less hostile than his words), all he does is bring stuff to Gamzee, and his text color is a little brighter than his blood color (Gamzee's a little darker).

So maybe, when confronted, he spills the beans. He knows that LE will rise, he knows LE will be defeated, and maybe he saw endless Meulins getting obliterated and when he's asked openly by his moirail, he just spills the beans. And they do it together. Why do I think that? There's a great video of TexTalks on youtube explaining that Rage is the aspect of Unreal and Absurd, and that's the only way, by stretching the bounds of reality and doing the insane and illogical, to send psiioniic powers into the future to empower the last Mituna standing. There's no canon way to do it, so they go offcanon, using Rage. And since Prince is a class of weaponizing the aspect (Eridan's blasts of hope from the wand, Dirk's soulsucking lightnings), they must hurt Mituna in order to do this.
The biggest hint is that there's a trinity that wasn't healed by the dreambubbles. Mituna, Kurloz and Meulin. So they're all somehow connected in this.

And why aren't Kurloz and Meulin healed either? Well:
Heart is the splinters. Heart. Is. The splinters. As in many, many splinters working together to empower one. The Prince of Heart is splintered himself, because Prince and his aspect working against him. We've been over that. They're channeling the powers of all three for this.

Mituna because only his powers can save everyone (except himself, he doomed).

Kurloz because that's the only way the psionic powers can cross time and space, and scratches and dreambubbles to reach the final!Mituna

Meulin because that's the only way all those powers can combine within the final!Mituna, like splinters coming together as a whole.
And the tradeoff is, they use their own powers and dreambubbles' healing effects [Rage, man] to maintain the transfer of powers for so long. And it only takes a part of Kurloz' and Meulin's powers, but the entirety of Mituna's, so he's to overexert himself to save everyone.