About Mulciber tried to use dark magic on Mary, Snape did not attend, he only heard from Mulciber. So if Mulciber said it is just a laugh, then Snape believed. Like Lily, about werewolf incident, Lily did not attend, she only heard from James. So she didnt know about the real reason why James saved Snape, she only heard James said how brave he was, and she believed.