I only saw Charla "in her ear" ONCE. There may have been more times that we didn't see on TV.
I think Des switched sides when she saw the balance of power shifting. When she arrived, the Outsiders held all the power, having just ousted Keith and Beau. Desiree was smart enough to realize that she needed to take one side or the other immediately, to have fewer enemies for a while. So she sided with the Outsiders and eliminated Alex.
Once it became clear that the power was shifting back to the Originals (with the returning guests getting a chance to come back in the game), Des was quick to jump ship on the Outsiders and throw her lot in with the Originals. They welcomed her just to have sheer numbers (and one more member who was expendable). But when it gets down to it, and the Originals have to sacrifice one of their own number to keep the rest of them in the game, guess who will be sacrificed first? One of the Originals who's not an Original. Bye bye, Desiree, Tom, and Holly.