Istendil lasted until 98 atleast. Found an old log claimed to be from august 98 where he was still emperor(http://www.qhcf.net/forum-2000/logs/messages/18023.html). And I remember him running around killing people in Galadon when I were new and I started back in 97 or early 98.

Graatch, Challen and Amlaruil existed at the same time, since all three were in the same hell raid that were the first to kill Dispater. Atleast if we can trust Scarabaeus site.