I have voiced my concerns/doubts about CD numerous times, but I wanted to throw something out there.....
If someone is asked to do an arrangement or contribute some violin, tuba, kazoo </sarcasm> to a particular song, it seems reasonable to me that they could be presented with just the music of the song to add thier part to (read music minus vocals). This could be to prevent the "song" from being leaked, and to be honest the person being asked to contribute really doesn't need to hear "the words" to add their part in the context they are being asked to participate.
In this day and age, it would be feasible to provide just the music to that person, and put the words back into the song when that was completed.
So it would explain some of these "rumors" as the person would have heard the music tracks without vocals, and tell a friend "Hey I added / did such and such to a track" to which the person extrapolates "damn the vocals aren't done".....