Yet again… For the hundredth time.
Doesn’t matter
Doesn’t matter, could be reforged again
You don’t know that, and he is no needed as we already forged Shadowmourne
You don’t know that, and it is no needed we can take by force
The reason is more customization for players
That was the reason they made man’ari demons and undead elves, there was no reason to add other than player customization
The inlore reason is created after the idea is going to be implemented, and there i many possibilities.
The reason is literally giving player more customization
You want a lore reason of why a death knight, searching for power, would go into a world where the scourge won to beat their Lich king down, save the survivors of this world and grab his sword? hummmmm i can’t think of ONE reason for it 
It didn’t exist before, just like you are whining about “t-t-the sword was reforge and doesn’t work!1!”" like that is an immutable status
Yeah, because we going to other timelines is not friendly, never happened before 
pot meet kettle
And it ends with a classic pigeon chess, they never change