I never said they're authentic, I said they're stereotypes.
And about those crowns, the first one is closer to the ones you'll find here (cap crown and blue crown):
http://www.touregypt.net/featurestories/crowns2.htm
Shapes may slightly vary in time, their history ran for millenaries, so obviously the stereotype won't be an exact copy of all of them. The cobra being the most obvious part of it.
I agree it's a mix of several crown types, but it does not refer to anything else than Egyptian stuff.
Now about the lack of references to cultures mummifying their deads, I agree it's missing, but mummification isn't a culture, and there are necromancers in the Heroes universe, so as long as nothing is told, it can be assumed that necromancers and mummies are related. The necromancers would be enough to explain why there are mummies, but not why they're disguised in Egyptian, so mummies in a neutral style can fit without any other background when mummies disguised in Egyptian can't.