This could all be solved easily by having a contract with the singer stating the wishes of the "contactor".

If you want it sung in the straight melody, state that in the contract with the stipulation that if the singer deviates from that, they return the full payment + 50%. A recording could be made of each performance and used to show if the singer took too many liberties.

If a contractor is willing to allow a few liberties with the melody, they could state that also and request an MP3 of how the singer plans to do it, subject to approval.

This may open up a mini minefield, but it would also make singers take the whole thing more seriously.

For me, I'd rather hear it sung "straight" from the original melody.

Bob
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http://soundcloud.com/bobcflatpicke/music-in-the-mountains