Legion and chronicles 2 stated that Sargeras power that Aegwynn aimed to lock in tomb of Sargeras was the body of the fallen avatar which makes Eye of Sargeras probably same level as or lower than Scepter of Sargeras. Which makes Titans spirit easily stronger as we see them in Antorus about the titans spirits.
It was stated by Malfurion in the frozen throne not Medivh because.... If you know anything about the lore you should know Medivh wasn't seen after the end of Reign of Chaos and Illidan put 4 nameless nagas to do it.....
He did it to Khadgar and Lich King body wasn't that well magically proctected as we have never seen him having strong magical protection.
It was stated she gave medivh her guardiang powers. Guardiang powers are by defined the powers that the council gives the Guardiang.