I'm fairly sure that when you say "the destruction of the Scarlet Crusade by the Lich King" you're referring to the DK intro quests. And I'm also fairly sure that those quests took place around the time of the WotLK pre-patch event.
Reason that I think that that is the case? Becuase during the DK quests you find a letter that says that Naxxramas has disappeared.
Here's the letter:
And if I'm not mistaken, later during the intro zone you find out that Dalaran has moved to Northrend too, might be mistaken though (should really replay the DK starter zone now that I'm thinking about it).To High General Abbendis
New Avalon, Scarlet Lands
High General Abbendis,
I hope this letter finds you in good health. My name is Dansel Adams, hunter by trade, Scarlet Crusader by blood. I am writing this letter to inform you that I have witnessed a miracle.
During a routine purging of heathens in the Scourge infested Plaguewood, my party and I were interrupted by a deafening screech coming from the sky. When we looked to the heavens, towards Naxxramas, our jaws dropped to the floor. I tell you this now, High General, 'twas a sight not to be believed! The dread citadel was moving. Slowly at first and then with a thunderous roar - poof - it was gone!
That's right, High General, Naxxramas is gone! No more! Could it be anything other than a miracle of the Light? I say not!
May it be that the impure, unkempt barbarians, are judged and sentenced next. Praise be the Light!
Please let our brothers and sisters know of this miracle, High General.
May the Light preserve you for all eternity so that you may spread the good word until the end of days.
Respectfully,
D. Adams
"The Closer"
Naxxramas and Dalaran was removed during the Zombie Apocalypse patch, which took place after the Sunwell had been restored and Kil'Jaeden had been flushed down the toilet. Not to mention during the DK zone the Scarlet Crusade sets sail to Northrend as the Scarlet Onslaught.
Wouldn't make sense for the SC to become the SO and set sail towards Northrend before TBC.
Therefore we can conclude that the Death Knights joined their respective factions right before WotLK, but due to gameplay reasons DKs still had to go through Outland.
As I said earlier, DK's and Pandaren characters have the same timeline problem, so why couldn't that end up happening for a Demon Hunter if they're added as a Hero Class similar to DK's?
Let's say that the expansion after WoD features DH's as a Hero Class, the start as level 85 and go through a leveling zone (like the DK one) and get to level 90 by the end. Then the Demon Hunter has to go through Draenor and get to level 98/100 (whichever they decide to do) and then the Demon Hunter can continue onwards to the new "continent/planet/whatever."
Through gameplay the DH goes through Draneor and helps clean up the Iron Horde mess that Garrosh created, but in Lore not a single Demon Hunter went to Draenor, but instead they skipped from their starter area to the place "Rise of The Burning Conquest: Reckoning: Revengeance" takes place in.
Oh and another little gameplay over lore timeline inconsisteny I just remembered. All races (except for Worgen and Goblin) can become Monks. Those Monks are trained by Pandaren, yet they still go through the Cataclysm leveling experience and the Pandaren didn't join the Horde/Alliance until the start of MoP.