Yeah, it’s long, I know, sorry not sorry and all that. And, before I begin, I’ll be clear in that I know there is no answer to this. We can’t possibly know how much was planned at the last minute, how much was edited in at the last second, and how much of an idea there was to explain Loki’s non-death that we’re just not seeing right here. It’s possible they had an idea, it’s possible that they had zero idea and just said, “Fuck it! He lives! We’ll figure out how later!”, and it’s even entirely possible that they just wiggled their fingers and said, “He lives because of magic! Reasons!”
Going down this road is entirely possible to just be an exercise in frustration because there may well be no answer at all possible, no matter how much you look at the details we’re given, and I entirely recognize that.
I’m going to do it anyway.
Okay, here’s my theory on what I think happened to Loki. But first I want to establish a few shots of what happens when one becomes one of the Kursed–they have that stone that’s crushed in their hand or shoved inside them and they begin to burn up in the transformation. We never really see what it does to their skin, because the two transformations we fully see, the person is wearing a mask. With Algrim, we never really see what happens to him, we just see him as Kurse later. We can see it does something to them, that it transforms them entirely, that the process gets under their skin and seems to burn.
When the renegade in the Asgard prison transforms, we see that he can lift two of the Einherjar guards and start burning them up from the inside just by touching their throats, which makes me think that just touching the Kursed skin is enough to start either transferring some of it or causing you to burn up as well.
Later, on Svartalfhim, we see Loki stab Kurse and there’s very definitely blood dripping out of him (as there was not when we saw Kurse with Frigga) and on the blade that he was run through with, the same one that then impales Loki. Though, the first we see turning into that odd mottled skin is Loki’s right hand… which is the one that hooked the implosion bomb (or whatever it is) onto Kurse’s hip, which may be why that turns first–whether contact with Kurse’s skin or because he reaches out and gets blood on it first.
I think that’s what the mottling was about–the same thing we saw happening to the Einherjar guards in the dungeon, only on a much lower scale because they didn’t have the same amount of direct contact or what have you.
Which probably would have been left there, but, well, Loki has to survive Svartalfheim somehow, right? The only theory with in-universe elements that I’ve been able to come up with so far is that maybe Kurse can burn people, but if you get some of his blood in you, it tries to go for the transformation instead, which Malekith notes will keep the person alive, pretty much no matter what, until they finally get used up.
If the Kurse blood being inside Loki (just like the stone gets inside the original person’s body) was enough to keep him alive just long enough to heal from that wound and then purge it from his system (whether via healing magic or he held on long enough to get back to Asgard to do it, whether before or after he confronts Odin, there’s a bit of time to work around whatever’s necessary) because it’s not a full transformation, there’s not enough of it to pull him all the way over, just enough that it keeps him alive/is killing him in a different way until he can fix it, then….
Then I can see a situation where Loki’s death scene was entirely honest, he genuinely thought he was dying, maybe he did die, but then came back through no plans of his own, just sheer dumb luck, and took the chance that was put before him.
I recognize that the answer is probably ultimately going to be, “Loki planned it!” or “Loki used magic to fake it when he saw his chance!” or something similar. But *shrug* I do like this theory, too.