I think you have to try and understand the person under the old covenant; they were not a spirit-filled Israelite, so they could not possibly have the mind of Christ.
That would be a good discussion in and of itself...
"Were the OT saints born
I'm going to assume this is your dichotomy:
obligation to the law (old marriage)
vs.
obligation to Christ (new marriage)
....If so, this is a false dichotomy as Paul isn't making this contrast in the portion of the letter you originally referenced (Rom 7:4). His contrast is as follows:
obligation to sin (old marriage)
vs.
obligation to Christ (new marriage)
There never has been an obligation to sin, it is to the law and couldn't be clearer using God's Word rather than man's reasoning...
Or do you not know, brothers—for I am speaking to those who know
the law—that
the law is binding on a person only as long as he lives? For a married woman is bound
by law to her husband while he lives, but if her husband dies she is
released from the law of marriage. Accordingly, she will be called an adulteress if she lives with another man while her husband is alive. But if her husband dies, she is
free from that law, and if she marries another man she is not an adulteress. Likewise, my brothers,
you also have died to the law through the body of Christ, so that you may belong to another, to him who has been raised from the dead,
in order that we may bear fruit for God. (Rom 7:1-4)
For
through the law I died to the law, so that I might live to God. I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh
I live by faith in the Son of God, who loved me and gave himself for me.
(Gal 2:19-20)
The reason death to the law was necessary...
The sting of death is sin, and the power of sin is the law. (1Co 15:56)
The context begins in Rom 6:20-23.
Rom 6 is all about how we are dead to sin through our being in Christ, no argument there.
Paul ISN'T saying the believer is free from the law.
This gets into what one means by law. As believers with the new birth we have God's law written afresh on our hearts. Jer 31:31, that is we have received a new nature which is a reflection of His nature, which was codified in stone given to Moses. Our new nature desires to live in accordance with God's law (nature).
It isn't a big if as long as one uses the faith and Grace and Spirit given as Rom 8:13 says.
We don't 'use' God in any way. Remember, we are the clay, He is the Potter.