The whole thing was a sham.  The witch set it all up so she can punish Drogo and murder little Rhaego.  Living comatose is worse than death.  The witch felt death was too good for Drogo and she planned to make him suffer.  Rhaego's was a quick death. Agreed.  People forget that the Targaryens only became kings three centuries ago.  A mere moment in the scheme of things.  Old Valyria did not have kings.  This vision lends credence to my belief that what Dany saw in the House of the Undying came from herself.  She has the ability to see the past and the future.  The warlocks did not.  They were using illusions and hallucinogens but that is the extent of their powers.