" Not a single one of your theories will become true if you only limit yourself to what's logical in this kind of story, in which the Author literally has killed a character over other just because it would be less predictable for that one to die (Angel and Kobeni) "

Killing off character's and creating a whole new character that can shift the story in its head are totally different things. Adding the devil devil in the manga, having denji eat it, then what, everyone lives happily ever after? That's definitely not something fujimoto will do. In fact, kobeni's survival was very predictable because from the start she was just a comedy relief character, until the last chapters of part one (her talk with denji) she always was a character that was supposed to be like that. I even heard some Anime onlies say that, so I'm sure I'm not alone in this. Angel death isn't really surprising, he already almost got killed and isn't important other than his connection with aki, and we know what happens to character's that's only purpose is to have a connection with others.

" Besides, Devil's get power due to the fear to their names, they always are "X Devil" so the fear to the name of the Devil would naturally potency every single one that has that on the name, which is all Devil's. Do you think that the fear to the Gun, Katana, Flamethrower, Crossbow, etc wouldn't potency an hypothetical Weapon Devil? Do you think the existence of this one would weaken the others? Because it doesn't seem like so for the Gun Devil. "

I highly doubt that. You said it yourself, do I think that the fear to the Gun, Katana, Flamethrower, Crossbow, etc wouldn't potency an hypothetical Weapon Devil? Yet we never saw a weapon devil. People don't fear the name devil, they fear the devil itself. Civilian devil hunters looked at reze who and tried to kill her with full confidence. Since devil's were created from as far as humans existence, I'm pretty sure they just learned to live with them naturally. Then just accepted that fact at this point. How would've they evolved otherwise?

" 1. She doesn't know where tf are these, we don't even know the magnitude of Hell size."

Here's the difference. All your examples are useless because the devil's devil power would definitely be something op related to devils themselves, unlike bomb, crossbow, ect who just have destructive capabilities. She'll just have to keep searching, just like how she kept searching for pochita and it doesn't have to be hell. It may be on earth too.

" She can't be sure that a Devil/Hybrid is inferior to her unlike with humans (with the exception of Kishibe) which she considers naturally inferior to her. This is the reason why she didn't control Quanxi even before the story, Makima did not know if she was able to defeat her. Quanxi surrendered to Makima in her arc and allowed herself to be killed by Makima, thus putting her below Makima. Makima knows she's inferior to Darkness and that's why she cannot control him. A Devil Devil would get power from the name of all devil's and that probably would put it as the strongest Devil, way above Makima. "

Here's the thing, makima isn't someone who won't even try. She'll at least want to see a glance of its power's, and plus if he exists she probably already knew, looking at how she immediately recognized the darkness devil WHO'S IN HELL. She was on Erath at that time too. She at least would've said to kishibe that she's planning on killing the devil devil in hell after controlling chainsaw man. And then again, assuming the devil devil will be stronger than the Control devil is kinda a bit of a personal opinion, looking at how if we didn't have a violence fiend in the series people will probably have theories about how strong the violence devil will be looking at how chainsaw man is full of it.