"There was considerable agreement on both sides that slavery would never flourish in New Mexico. "
I now believe this "We had to prevent EXPANSION of slavery was just made up astro turf crap to prevent congressional representation that would ally itself with the Southern states. More representation in the Congress would upset New York and Washington DC's sweet deal where the bulk of Southern export profits fed the appetites of both cities.
No real possibility of slavery expanding into the territories was ever going to happen.