They figured out that to keep the severed necks from regenerating, they had to cauterize the wounds to make its body think there was nothing to be healed, thus canceling out the regeneration of the heads. But when they attempted to cast fire-magic from afar, the other heads would constantly get in the way and block the fire-magic from hitting its target, so they had to get in close-and-personal to cast fire-magic as soon as the neck got severed.
No, a week or so back now MangaDex went through a deletion phase where they deleted a crap-ton of chapters, I believe it was to increase the legitimacy of the website for whatever reason, I don't know the specifics, but I do know there was a reason for the deletion that had something to do with following laws and stuff.