Shadows of Rage do not spawn in the Sullon instanced..at least I have never seem them spawn during raids. During samples of corruption task however..you will see them every 20-30 minutes.
Untrue, this npc does not spawn upon someone's death. It spawns when someone succumbs to the rage of the tower after a check is run every so often during the mission.
Segnar also gets stronger for every person who dies, hits around 1100 1200 to start, up to 2k if everybody is dead. Which obviously doesn't matter a whole lot unless you retry him. He spawns at least 2 adds during fight. During the mission you have a total of 4 trials to complete to win. Didn't go past first trial. You can't have more than 10 total deaths during the event or you lose.