Re: Downplaying Mulciber's actions
We have canon evidence that sometimes non-Dark magic can have worse consequences than Dark magic (imagine Severus accidentally Stupefying George and making him fall from the broom rather than cutting his ear off with Sectumsempra). And we have canon evidence that some of the things that pass for 'jokes' at Hogwarts are incredibly cruel. So I can see Mulciber's 'joke' as equally bad to what the twins and the Marauders got up to on a regular basis. And Lily being either naive or hypocritical for thinking it worse because of the Dark Magic aspect.