There seem to be exactly 2 Sinuous Adders as PHs for Sable. This would correlate to the other named of which some have also 2 PHs. The Adders Egg is only dropping from these two. We tried to get the quests in our guild as a 13 people raid fast but because of the respawning of the two PHs this was a bit a timesink. Farming the most other parts for the quests where fast and easy with big pulls and AEs but this one was a bit longer....
We cleared at first complete parts of the zone to get them spawnd. All adder types snakes all vipers... nothing. So this seem to be the case. Good Luck.