A beginner's questions about WC3 lore

I played WC3 RoC and FT campaign for the first time (Normal difficulty ofc) and managed to finish the entire story. I think the overall story was entertaining. But the story also raised a few questions. I would appreciate it if those familiar with the lore of this game can answer them. And sorry in advance for the long post.

1. Medivh said that he brought orcs into this world. How did he even know about the existence of orcs who are supposedly from a different planet/dimension? And why did he bring them to this world?
2. In multiple instances, it is indicated that the night elves alone defeated the Burning Legion more than 10000 years ago. No mention of help from other races are ever mentioned. Even Mannoroth seemed to acknowledge the power of night elves and Cenarius who supposedly defeated the demons single handedly. Then why this time, the help of orcs and humans as well as sacrificing their immortality and powers over nature was required? Why did Cenarius (who was supposed to be the god of the night elves) easily killed by mere mortal orcs? Hellscream drinking from the corrupted fountain shouldn't have made any difference given that even Mannoroth himself had failed to defeat Cenarius before.
3. Why was Illidan imprisoned for eternity? The story makes vague assertions about his addiction to power and magic but what exactly was his unpardonable crime that he was sentenced to eternal imprisonment till Tyrande freed him?
4. Medivh's plan to save the world was to convince orcs, humans and night elves to join forces and make their last stand on Kalimdor. To do this, he succeeded in convincing the orcs but humans rejected him thrice. So now he approaches Jaina with the task. However, it was Medivh's own spellbook that brought Archimonde into the world. Archimonde who had never been on this world for over 10000 years, knows where the spellbook is. So it's unlikely that Medivh doesn't know where his own spellbook is. If he asked for the spellbook to be destroyed, then the Burning Legion would never have a chance to invade the world in the first place. The undead and the dreadlords might still be a problem but they aren't seen as a major threat at all by Medivh. So why not stop the invasion by destroying the spellbook? Certainly that would have been much easier than convincing two enemy races to sail to a distant land and co-operate with yet another enemy race.
5. Medivh said that he was killed by humans for bringing orcs into the world which also resulted in opening a passage for demons. If so, then who resurrected him? He couldn't have done it himself. All he says is that he has come back. But exactly how did he come back alive? Even if we grant that powerful heroes of each race can be resurrected at their altars, this doesn't apply to Medivh since nobody seems to know who he is.
6. The hero trio of night elves seem to make decisions that make no sense. Tyrande does things that goes against the best interests of her own kind. Killing the elf wardens of the prison or initially opposing any alliance with the humans and orcs when they are shown to be fighting against the undead. Worst part is, most of these decisions seem to be motivated more by her own pride than rationality. For his part, Furion seems to bear endless hatred towards Illidan for becoming a demon but unexpectedly trusts and forgives him for saving Tyrande from the undead in Frozen Throne campaign. But Illidan's exile is still under effect. Illidan uses the Eye of Sargeras to try and kill the Lich King but doesn't inform this to Furion or Tyrande. If he did, they would have helped him in his quest or even found a more acceptable way to achieve that goal. Aren't they supposed to be the oldest and wisest beings currently alive in the world?
7. The naga that Illidan commands reveal themselves to be former night elves. It seems that some disaster long ago had transformed them into the naga. Both the naga and the night elves seem to loathe each other. What exactly happened which led to this?
8. In the final in-game cutscene where Illidan and Arthas fight, Arthas wins rather easily. This is despite his own powers being weakened due to the rupture of the frozen throne as well as Illidan becoming more powerful after consuming the skull of Guldan. Illidan also has much more combat experience than Arthas, having lived much longer. Is there some reason why Illidan despite all his efforts, was no match for Arthas?
9. Who is the demon Kil'jaeden? Is he another leader of the burning legion? Does he rank above or below Archimonde? Now that the Legion is defeated, what is the use of trying to get Illidan to kill the Lich King? And if he can find Illidan all the way in Outland, why did Archimonde need to be summoned to the world?