Hotep is working for Carmen Sandiego
How else could he be able to steal the Nile River? The tomb was already unsealed
In the film, Amelia Von Butch appears to unseal the tomb by blasting it open with explosives, yet somehow the army of the undead was already down there waiting for the tomb robbers. How did they get down there if the tomb was sealed? Also, when the door is destroyed wind appears to blow out of the tomb. Since it turns out there isn't any actual curse, there was probably a wind machine hidden behind the door to make the hoax more convincing.Omar and Velma probably removed the real door to the tomb and then used the quick-drying cement to make a fake door, which they then decorated by writing the "curse" on it. This way they could put their plan into action while keeping the real door safe from destruction.