Christ is one with the Father, so he should not be interpreted as being in disagreement with what the Father has commanded, but rather John 8 is an example of him acting in accordance with what the Mosaic Law instructs. There was no judge to pronounce a sentence (
Deuteronomy 19:17-21), there was no man accused (
Leviticus 20:10), he didn't have any witnesses to examine (
Numbers 35:30,
Deuteronomy 19:15), and he did not have a confession, so if he had condemned her, then he would have acted in violation of the Mosaic Law. Just a few verses later Jesus said that he judged no one (
John 8:15) and he also said that he came not to judge (
John 12:47), so he did not exercise authority as a magistrate and did not condemn her, but he did recognize her action as sin, and told her to go and sin no more.
In Galatians 4:4, Jesus was born under the law, so he was obligated to obey, and he was sinless, so he never broke it, so again John 8 should not be interpreted as Jesus acting against it and thereby sinning. The New Covenant was not established until Christ's death, so everything that he taught during his ministry was how to live as someone who is under the Mosaic Covenant, and he did not establish the New Covenant for the purpose of undermining anything that he spent his ministry teaching, but rather the New Covenant still involves following the Torah (Jeremiah 31:33). God is sovereign, so we are obligated to obey His law and to refrain from sin regardless of whether or not we consider ourselves to be under it.
If we look at Colossians 2:16 by itself, then it is ambiguous in regard to two possible situations:
1.) The Colossians were not keeping God's feasts, they were being judged by Jews because they were not keeping them, and Paul was encouraging not to let any man judge them for not keeping them.
2.) The Colossians were keeping God's feasts, they were being judged by pagans because they were keeping them, and Paul was encouraging them not to let anyone judge them for keeping them.
If we look at the context of what Paul described of the views of the people who were judging them, then it becomes clear that the 2nd situation is the case in that they were being judged by pagans, such as saying in Colossians 2:20-23 that they were promoting human precepts and traditions, self-made religion, asceticism, and severity of the body, which is against celebrating God's holy days. We must obey God rather than man, so we should be careful not to mistake what was only said against obeying the teaching of men as being against obeying the commands of God, especially when the point that Paul was making was that we should let anyone prevent us from obeying the commands of God.