Sea,
Actually, I shouldn't be so sarcastic, especially since you offered the theory in good faith and you are very articulate about it. I don't have any counter evidence except the fact that I never heard the theory, I never heard of the investigation of the theory, and Eva's posthumous recordings were really not that popular in the USA when compared to truly successful recording artists. Her wonderful singing and the unusual and tragic story have captured the hearts and minds of a few here and there, but it is a relatively small viral marketing phenomenon in the USA.
So in that the alleged conspiracy failed miserably, and in that hundreds of people in the Wash DC area knew her personally and in that people visited her on her death bed, and in that big crowds of people witnessed her last performance in public at her own fund raising benefit, it is hard to believe that so many people could be enlisted into such a conspiracy and be kept quiet.
I put this conspiracy theory into the same category as the 9/11 Pentagon plane theory, where it is proposed that it was a missile and not a plane that hit the pentagon. The problem with both of these is that there are too many average Joes involved in the events to suggest that they could all keep quiet.