The only known lore about pandaren visiting the Eastern Kingdoms was one single pandaren, Chen Stormstout, who also caused Brewfest to become an annual festivity. One pandaren =/= multiple pandaren
The third war ended with the death of Archimonde at the World Tree, the dwarves arrived in Kalimdor (with the rest of the survivors of Lordaeron) way before that.
One known settlement is still more than none.
Again, you're assuming that Acherus arrived shortly before before the player DKs were resurrected. What if the blood princes had time to get Arugal BEFORE the DK starting zone happened?
How were the forsaken able to control themselves after they were no longer under control by the Lich King?
I'm not sure why you're bringing the gilneans and the Alliance into this, as it's a know fact that the death knights were only allowed to join the Alliance because of the letter written by Tirion. Race didn't really matter in that case. If there were pandaren death knights, they'd be accepted because of Tirion's recommendation, not because of the fact they were pandaren.
So it would make sense for two pandaren to travel into a scourge infested land to ''searching for ingredients and exploring''. Yea right.
Two worgen escape from Shadowfang Keep/Pyrewood Village, they weren't feral because they showed to already be somewhat in control. (but I guess you ''magically'' don't remember the worgen in Pyrewood Village showing up as humans during day), make their way to the Argent Dawn.
(because they probably heard about 'em, and figured that'd be the best idea to fight against the Scourge)
And to throw in another assumption, what if the blood princes killed some worgen at Pyrewood Village, when resurrecting Arugal? (followed by using said worgen corpses to create worgen death knights)
I think I'll go with the worgen one
Source?
I feel like you're just grasping at straws at this point, to validate your own theory of why pandaren death knights should exist.