That's what I'm talking about!
Now ... if Illidan had come first ... he would have walked up the stairs, and herotically shattered the Frozen Throne, along with Ner'Zhul's soul. Arthas would've yelled "NOOOOOOOOOOOOO!!!!!! YOU SON OF A BITCH!!!" and then either: A: Go hunting for Illidan. B: Stay in Northrend, in exile (most undead would ... die I guess ... or become free-willed). Or C (this is far out): By killing Ner'Zhul, his spirit in Frostmourne would die too, the blade would shatter, and release
his own soul, thus making Arthas nice again. Arthas would then return to Stormwind, to get sentenced to death, to lifetime in prison, or be crowned as true king, and then assemble an army to crush the Forsaken, or become neutral/allies/enemies-because-they-become-Horde. What heppens then with them, is up to you.
Illidan would've completed his Master's task, therefore not needing to fear his wrath, or suffer from the grievious wounds Arthas gave him, and, bevause of that, stay sane. Maybe (!) he would even treat Akama and his Broken comrades better, and avoid being betrayed. Anyway, the world of Azeroth would be a better place to live, the fight against Illidan quite harder (if we decide to kill him), and the latest expansion much more boring.
For the other question: If Kil'Jaeden hadn't come along, the orcs would probably still be living on the uncorrupted Draenor, in relative peace with the Dranei. If Kiljaeden had come along, but Nerley had discovered Kil'Jaeden's lies in time, I think that the orcs would be in war with the Eredar, but get help from the dear Dranei, and in the end, they would be great allies, and that was it for them.
However, in the human land, there would've been no Lich King, nothing to cause the third war and corrupt Arthas, saving countless lives - King Terenas could maybe even still rule. However, if the Burning Legion had taken a different approach, and said "fuck the humans" and gone for Kalimdor in the beginning, things would've been looking grim:
No orc corruption, no first war, no killed Medivh, noone to warn the humans of the invasion, and as said in CoT; no first war, no Alliance, civil war. - remaining force against the invasion: Night Elves and Orcs/Tauren
On Kalimdor: No first war, no orcs, no known Thrall, no immigration, noone to save the tauren, who would've been slaughtered by the centaurs. On the bright side, Cenarius would live. Remaining force: Night Elves and Cenarius, but he ain't THAT powerfull.
On the other hand; a large part of the invasion was undead, but I'm sure the nice Eredar leaders could've filled the spots with pure demons.
Conclusion: If Kil'Jaeden hadn't owned Ner'Zhul retorically, World of Warcraft would've been dead. All of it.
If you've read this long, you deserve a cookie.
I ain't asking no question!!!!