I know that I have already told you that hiring the right demo singer is the smartest demo money you can spend. I stand by that. There is a huge difference between someone singing your song on pitch and someone delivering, interpreting and selling your song to the listening audience.
Though you have every right to expect a demo singer to stick to the melody you have written, I encourage you allow them some latitude to use their instincts to sell your song. This is especially good advice if you are a male songwriter who has written a female song or visa-versa. The Mars/Venus rule applies here. Vocal tricks and techniques that work well with the male delivery may sound contrived and unbelievable coming from a female vocalist.
Cast a vocalist who you believe can reach the pitch market you are aiming for, give them some guidance about the attitude and emotion you are going for and then sit back, at least on the first run through, and see how they interpret your song and directions. You may be very pleasantly surprised. They will almost surely come up with embellishments that are new to you. If you like them, your song is all the better for allowing the exploration. If you don’t, you can have them stick more closely to what you sang on your work tape.
As songwriters, most of us will lay our songs at the alter of artists, asking them to interpret them in a way that will sell them to their audience. Try allowing your demo singer the same trust and see if it breaths new life into your song demos.