Void is the absence of Light, not the counter-reflection of Light, which is Shadow. Therefore, nobody can prove that the WoW universe conception is about the bipolar nature of Priests. I'd say Priests refer to the Shadow connecting those ends, not the absence one of those. That, however, does not render the Priest as a heir to that power just because Metzen hinted stuff about the origins of the universe.
As for my claims regarding proving the origins of Shadowform, I've said any proof I can provide is vague for the time being. You, however, try to disprove the TCG claim by saying it is non-canon. I mean, the cards have been released throughout different periods of WoW and the criticism regarding this controversy has arisen relatively recently.
An additional note would be this quest: http://www.wowwiki.com/Quest:The_Stolen_Tome
Pay attention to the context, the fact that it was present past-Wotlk when Priest did not have all those "void" skills and that along with all the other Warlock quest mentioning the Void and its connection to the powers wielded by the Warlocks. I do not deny the fact that from MoP onwards Priests have gotten Void related abilities and the Sha. All I'm saying is that this was not the original idea and there is proof about it (see Warlock quests), even if the Warlock Shadowform or Warlock void cards turn out to be baseless points.
If Priests are to be the prime wielders of Void/Darkness, it would be nice to know while its early in order to finally focus in a single class after all these years. Namely, Priest.