I'd like to point out that its just one of several theories that Hashut is a Khornate Daemon who rebelled against Khorne. To my knowledge there is no official fluff from GW regarding it.

Second Malal is not possible to include. GW don't use him, and indeed don't have the IP rights to him. And since we're making a fan product of the GW universe, I doubt that we can include him.

The former Everchosen of whom you speak would probably be Be'lakor, who however wasn't the first Everchosen, but indeed the first Daemon Prince. Also I am unsure if any Everchosen have ever manage to reach Daemonhood, they have all to my knowledge been destroyed by a Champion of Light.