First of all, I am going to say this right now: I am not even remotely certain of this. I think it was a very fun thought experiment, it might be partially right, or maybe I'm looking into things too much. That said, I am going to explain what I feel is a shitload of stuff in One Piece.
First of all: One Piece is literally everything. This is a roundabout explanation but to summarize:
This I think, is the first piece of the puzzle. The narrator said this, and I think we can agree the narrator is an absolute authority. Secondly: A physicist said that anything humans can imagine is possible. This is not a philosopher, or anything like that, someone who deals in cold, hard facts. Again, assuming absolute authority, this statement is true.
How?
All matter can be changed freely. Awakening is a piece of the puzzle, Doffy can turn some matter into strings. Devil Fruits turn peoples bodies into different types of matter (metal, rubber, etc). As it stands, matter can already be manipulated. But what makes matter?
Unfortunately, here's where things get a little tricky. Everything seems to have a breath, rhythm, voice, or wavelength. How these things are interchangeable or the same, we don't know, but the idea is simple: every form of matter is expressed as a wavelength, similar to how say, light and sound are basically just expressed as different kinds of wavelengths, or matter is comprised of atoms, which, if you change the components of, can create other forms of matter. Zoro, during this moment, understands the fundamental components of the universe when he is closest to death. he can hear them speaking to him, or at least acknowledge them. We've also heard that seastone disrupts the wavelength of devil fruits (or has a wavelength that disrupts devil fruits). So awakening is basically, you can hear the voice/wavelength/breath of your source of matter, and then change anything to express it in that fashion, making it sort of the penultimate version of a real Buddhist style awakening.
Haki, by extension, if your will changing your body to be immune to the effects of say, devil fruits, or not getting hit by a devil fruit, etc. With willpower, you can manipulate matter, even with out a devil fruit. So this goes into the original point, humans can do anything they image, literally, through the power of will. Matter is changed by will. Here is where things get stranger:
So, someone who is truly awakened and powerful enough, theoretically, can change everything. Someone with this form of power would have the greatest freedom ever. This is why Luffy is our protagonist, look at the content of his character, what would he do if he, in all essence, became a god? What would he do? He would probably change the world for the better in some capacity, but he would also not like, destroy it, rule over it, etc because he wants to just have fun and eat meat. So Luffy is SUPPOSED to be the one Roger wanted to reach One Piece. Imagine anyone else with that power.
Why are both of these things relevant to each other? Because if anything that can be imagined can happen, and it is done through changing the very foundation of matter, and any kind of matter can turn into any other kind of matter, everything is comprised of the same parts. Basically, everything is literally One Piece.
So everything can become everything, big deal. If everything is comprised of the same parts. Nothing truly dies, it simply is changed. This is kind of like, basic circle of like bullshit, everything, in a sense, reincarnates. So what else reincarnates?
Will.
The Will of D is something that is inherited, will is passed down from person to person in the same way a soul can reincarnate, I'm not sure if wills are tied to souls or whatever. But basically, Willpower changes matter, Willpower is passed down from generation to generation. Luffy has literally inherited Roger's will, even if he's not Roger (which is important), and this is actually a similar Buddhist idea, part of you moves on, but you're not literally transported into a new body when you die or whatever. So Luffy gets Roger's will. If wills reincarnate, and if fruits reincarnate, and wills can change matter, then perhaps, just perhaps, Devil fruits are the willpower of something. Honestly? I don't know what. Actual devils? People? Who knows. But Luffy's body was transformed by the willpower of… a rubber devil? Maybe. This is rather loose. But again, if everything has a will, what if you knew what the will was?
That's basically what the voice of all things is. When Zoro is almost dead, he gets a glimpse into this. Luffy can hear people think, but what about Roger? He probably heard what like, actual stuff was thinking. That stuff had a will, and thats sort of what he heard. Why is this important? Achieving the ability to hear the voice of all things is basically the final stage of enlightenment. If you can "hear" all matter, all of its wavelengths, and then have the will to change it, this is what makes awakened Luffy so powerful.
So to branch off of that, when someone almost dies, and they sort of become aware of the voice of all things (similar to Zoro), The Ds smile when they are about to die because they know they aren't really going to die, and that their will will live on forever. they hear the will of other stuff and go "oh, huh, cool, I'll be just like that, sorta?" and then they die, and their will moves on. So the Will of D is sort of a predisposition to awakening, in some sense.
So why does killing two devil fruits kill you? It's two wills trying to change your body's matter at the same time, the wavelengths fight, and can't agree, so you explode. Okay, that was simple, sorry. But let's explore Blackbeard.
Blackbeard is a weird case, but I'll explore two brief theories:
One: Blackbeard has two souls.
Simply: Despite the whole wavelength arguing thing, I think that the power itself resides in one's soul (which dies upon actual death in some capacity). Brook died, and his soul can wander around, but when he moved back into his body, he still had his ability, beyond that, his ability allows his soul to leave his body. Blackbeard simply has two (or more) souls. This makes him unique in body, lived two lives, etc.
Second theory:
Blackbeard's darkness powers allow him to neutralize a wavelength back to its original state. IE, if you were a person, and you had the rubber fruit, the darkness fruit changes your rubber wavelength back to a flesh wavelength. What this means is Blackbeard cannot change his wavelength at all. In the sense that one might change their body's wavelength to fire, his wavelength changes to darkness, what is darkness? The absence of light, light is a wavelength. So if his DF is to change his body into no wavelength… then nothing happens. He's a logia, because his body is a thing, it just so happens that that thing is expressed as his flesh and blood. So when he says "I take more damage", it means he can't even use haki to block attacks! But at the same time, if his wavelength can't be changed, then he can become a earthquake creating man.
Aaaaaaaaaand some icing onto this whole set of weird ass theories: if everything is expressed as rhythm, or wavelength, what is a quake? It is the creation of a wavelength or rhythm. So literally, blackbeard's fruit allows him to negate and destroy and change literally everything, making him the antithesis to an awakened Luffy, only done through the power of devil fruits.
PS I have no idea why water beats Devil Fruits, but my guess is since the celestial dragons think themselves as heavenly, and Doffy said "the winners write history", since they are the winners, they called the devil fruits just that. How did they take over the planet? Well, they live on the HIGHEST POINT in the WORLD? right? And if water beats devil fruits, (devils being the enemies), they simply have the ability to FLOOD THE ENTIRE WORLD. That's why the world is comprised of so many islands and is mostly ocean. It was flooded to kill all the devil fruit users and win a war.
So, I'm crazy, but I wanted to write this all down so everyone can laugh at me or the 5% chance I'm right, I have successfully predicted a boatload of stuff.
BTW, I don't think this necessarily invalidates the other predominant theory about One Piece, where Raffetl is under the red line, reason being, the reason you could get to raftel in the first place was that the world wasn't flooded...
I am not necessarily saying everything here is something I believe to be 100% true, it's more of a piecing together of what we have so far, and trying to make sense of it. It is a thought experiment and I hope that maybe others can contribute and find either holes or further explanations in the theory other than "lol you're on crack dude".