idk if this has actually been discussed, but umm I believe there was mention of an actual DEVIL(a sea devil IIRC) in league with the origin of the Devil Fruits >.>
There was also mention of a rumor that if a Devil Fruit user is near another Devil Fruit, he'll explode. My point is, almost nobody in the One Piece world actually has a clue as to how Devil Fruits, and therefore, we don't either. We know from Chopper's description of the Rumble Ball and Smoker's explanation of Seastone that some sort of vibration or wavelength is involved, that once a Devil Fruit is eaten, a new one of that type won't grow until the eater is dead, that a person can't eat a second Devil Fruit without dying, that it's the first bite of the fruit that transfers the power and after that the fruit is useless, that they all have a swirl design on them, that they taste horrible, about the three types and how to divide them, and that's about it. And precious little of that is about where they come from or how they work, more about what they do.
We also know from Oda that Vegapunk knows a lot more, and that the Straw Hats, and therefore we, will know a lot more too once they meet him. But that's not for a while. Until then, the bit about the Sea Devil seems to be just a rumor that people made up to explain them, and therefore, the origin of the term "Devil Fruits".
"Hey, I ate this fruit and now I'm made of rubber!"
"Really?! How did that happen? It must be because of some sort of Sea Devil!"
"So that fruit was a fruit of the Devil... I know, we'll call them Devil Fruits!"
I think it went something like that. Some guy just suspected that the fruits were the products of the Devil, and the name caught on. But it was cast as just a rumor. It may be that that's really where they come from, but we have no way of knowing until the Straw Hats meet Vegapunk.
About the SBS about producing their element... Producing things (such as wax, or body parts) is a trait of a Paramecia fruit.
Oda was asked whether what Ace did for his "Flame Fist" move was create fire, or become fire. Oda answered that what Ace did was turn his arm into fire, but increase the intensity of the fire so much that it became huge. So Logias can't so much produce their element as they can turn their bodies into their element, and then increase the intensity of that. For instance, could Crocodile create a tornado of sand? No. Could Crocodile turn his palms into sand and start swirling it around until it becomes a sandstorm? Yes. And could he use sand surrounding him if he turned into his elemental form and incorporated that sand as an extension of his body? I would say yes.
Of course, Oda later made things complicated by having Ace throw lances and balls of fire unattached to his body at Blackbeard, but I've interpreted that as parts of his body that he seperated, and therefore, can still control, because they came from him. While Ace can't control fire, he couldn't make a bonfire bigger or smaller or anything, he can control the fire that is (not that comes from, that is) his body, even if it gets seperated off. And of course, the same would apply to all Logia users.
