Dear Gramps,
Why would the premortal Jehovah tell the Israelites that stoning was the mandatory penalty for adultery, and then not agree to its implementation when the woman taken in adultery was brought before him in mortality? Why wasn’t His leniency part of the ancient law as well?
Robert
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Hello Robert,
With regard to the woman taken in adultery, there was a proviso in Jewish law that 
the accuser had to throw the first stone. There was a second proviso that the accuser could not be a party to the adultery. Thus from a strictly semantical point of view Jesus was not arguing against the law. He was arguing FOR it, but demanding it be enforced in its entirely rather than in the selective and self-serving way that the accusers wanted it enforced. The fact that the accusers ultimately backed down indicates not only that they were stirred in their consciences; but that they knew they could not meet the evidentiary burden required by Mosaic law. Of course, Jesus, as Son of God, probably knew that from the beginning.
Also, some scholars claim that the Jewish leadership of Jesus’ day did not have authority to sentence anyone to death independently of the Roman authorities. By His response Jesus avoided speaking openly against the harshness of the law (which, according to Paul, was only a schoolmaster intended to bring mankind to Christ anyways), but also avoided making a statement that could be construed as usurping Roman authority and thus implicating Himself in a charge of sedition.
Finally, while a treatise on ancient Jewish law is beyond the scope of this answer, it is also worth noting that women accused of adultery (and people under sentence of death generally) under Mosaic law did have some legal recourse.
Sincerely,
Gramps
