I like your suggestion that hardcore addicts essentially become incapacitated by their addictions. Something that developed that the WW didn't foresee that effectively undermines their plans -- the elixir can undermine Siervage's control only so much.
I don't know about the conspiracy angle for why Siervage hasn't discovered the elixir. The inquisitor vampire would be the obvious one, since he is leading the investigation, but he doesn't seem one to conspire against Siervage, and it seems like fooling Siervage would be enormously difficult.
So far I'm inclined to rely on vampire arrogance -- sure, they have noticed that some spawn are acting strangely, but most of those are the spawn of murdered vampires, and it would just be inconceivable to them that a drug could release them from the control of their master. Also, since they are under siege they are not off exploring the other passages underground where the dealers and users hang out. Does that seem plausible?