A soul reaching the shadowlands is entirely unnecessary for death itself. The people of Azsuna were still all killed, their souls remained anchored to Azeroth. But they died regardless.
Sylvanas died at the hands of Arthas, then at Icecrown and finally a third time in Silverpine.
No it is not, any necromancer can bring back the dead, without such nonsense, heck powerful necromancers can bring back beings that have been dead for thousands of years without taking their palce.In Icecrown and in front of Gilneas, she truely died and it took Valkyrs' sacrifices to bring her back. A Valkyr had to replace her in the Shadowlands.
One soul for another, because this death is the only one that count in Warcraft universe.