Anything a Magic User does with magic cancels as they die, right? Does this apply to their (unused and stored) smoke as well? It seems like it must, otherwise Dokuga would have been left with useless smoke after he killed the healers. To be fair, it could be that they were just guarding the smoke, which came from others in their guild, but it seems unlikely that none of the smoke came from them (they were guild members who presumably specialized in healing magic). Also, Fridge Horror: If Noi dies, does every wound she healed except for ones caused by someone else's magic come back? Sure, she'd already be dead so it wouldn't really matter for her, but Ebisu would basically disintegrate, no? Or is healing magic, like curse magic, exempt from No Ontological Inertia?
Noi has actually died for a few minutes numerous times and nothing like that ever happened. It can be assumed that this kind of stuff happens when the Devil Shaped Tumor is destroyed, severely damaged, dies of starvation when it's magic user host dies or is taken out of the body (vide En's mushrooms rotting even if Kai preserved the Tumor and became it's new host.
Maybe the thing with magic reverting isn't reverting, but instead "going back to normal", in other words, only magic that change's the subject's normal structure is prone to disappear upon death, so Noi's, Nikaido's and Kikurage's magics are all out. Similarly, don't think there were any scenes of En's mushrooms going back to being people; so maybe it's not as much as "going back to normal", but instead "going back to a stable state", as so far (and as far as I can remember) the only magic shown to revert has been unstable magic that only changed part of the victim's body.
When you think about it, Asu/Kawajiri's magic does not make any sense at all. Every other moderately powerful magic user could do the same by summoning a door - even Ebisu could do it!
Some doors have a set location, specifically the ones in En's manor that are constantly guarded. Asu's power is quick and can be used in battle to catch his opponents off guard whereas doors are an emergency getaway. Not to mention it's Asu's power that allows Kai to get into the heavily guarded wing of the manor and kill En.
When Shin exterminated the Hole's anti-magic militia, why did no one form a new one after he disappeared? Even Kaiman and Nikaido could do that.
Does the title "Dorohedoro" actually mean anything?
It can be read either as "Mud to Mud" (Doro he doro) or as "Mud [and] Sludge" (Doro hedoro)