I don't think that fooled anyone. It was obvious from Book 1 that Snape, though sinister, was Dumbledore's man and chose right over wrong. Only the youngest of children may have thought Snape was actually on Lord V's side. Likewise, all the people who thought that Dumbledore wasn't really dead probably just didn;t read book 6 very well. JK made it very obvious that he wasn't going to make a "come back from the dead". When his portrait appeared in his office, or when Harry was thinking that he would never speak to Dumbledore again or have his bright blue eyes bore through, that should have been more than enough to let people know that Dumbledore's time with the living was over. Also, in the previous books she forshadowed this event and made it perfectly clear that even in the HP world, death was final...no comebacks. And if THAT wasn't enough, she even made the point flat-out on her website that Dumbledore was, indeed, dead and he would not be coming back to stand beside Harry.
Most veteran readers will aslo tell you that the best hero books will leave the hero completely on his own for the final battle. All of his crutches must be removed (Sirius, Dumbledore, Moody) before the reader can start to feel the desperation that the writer wants to convey before the climax. No one would fear for Harry if Sirius, Dumbledore, or Moody were still around...the reader would be thinking that one of these very strong a nd capapble wizards would come out of nowhere to save the day. But JK was smart and took away the "help" so that the reader knew that he was Harry or Voldemort, win or lose.