Larry,
I have no problem with what you are saying, but could you describe the "OT mindset", according to you? I think that Jesus is presuming that the Torah, correctly understood, was indeed based upon what we would call Christian principles, for the most part. Mercy, forgiveness, justice, love of neighbor and God. Faith and trust in God.
I'll try to ascertain what you mean, and tell me if I am correct.
In my opinion, I believe you are pointing out the difference between the religious self-righteous and the humble man of God who understands the point of rituals. Thus, the OT Minor Prophets often attempted to correct the "OT mindset" of doing external works, sacrifices, etc., while having an impure heart, men who continued to persecute the poor and helpless, the wicked of heart, but on the outside, "worshiped" God. Thus, for example, "I desire mercy, not sacrifice" to mean, "sacrifices are of no value to Me (God) if it does not breed an inner desire to show mercy to your fellow man.
Is this what you mean by "OT Mindset"? Maybe not.
Paul's zeal was misplaced, but clearly, he was NOT the type of man whom the Minor Prophets were describing, a person of external works only, but impure of heart. Paul took his love of God seriously, and his zeal vs Christians was legitimate, given his views of God - before being given a divine revelation that God had come in the flesh as Jesus Christ. But remember, Jesus says that the OT speaks of Him throughout...
Regards