There is a good case that kurios in Paul consistently refers to the Son, and theos to the Father -- at least there are many examples of this distinction and no unambiguous one against it. I let you appreciate what the substitution of Jehovah to kurios does to the Pauline texts. Even from a unitarian standpoint.
Not sure if I understand your point entirely, but an example from Paul show that "kurios" is most obviously a reference to YHWH is 1 Cor. 2:16. To go a step forward, it would be odd for Paul to use "kurios" as a reference to the Father here- when it is usually reserved for Christ. Especially when we consider the text he was probably looking at from the LXX, which the available evidence shows- used a form of the Tetra. Scholar David Trobisch also uses Paul as an argument for the DN in the NT at 1 Cor. 3:16-18, he writes:
"Paul sometimes bases his argument on a quote from the Jewish Bible and carefully makes a distinction between JHWH and Christ. After the nomina sacra are introduced, however, both may be respresented as ks, and may be interpreted as synonyms by the readers.I want to demonstrate the shift using the passage quoted above from 2 Corinthians. The text continues, "But when one turns to the Lord, the veil is removed." It can easily be documented that Christian readers tend to interpret "Lord" as a reference to the Lord and Savior Jesus Christ. Assuming that 2 Corinthians 3:16 alludes to the wording of Ex 34:34, and assuming that the exemplar of the Jewish Bible in Greek that Paul uses contained the tetragram, it may be safely concluded that the following verses contain the tetragram as well. If kurios is preceded by an article, the word probably refers to Christ, but in all other instances Paul's original letter showed the tetragram
2 Cor 3:16-18 (own translation) But when one turns to JHWH, the veil is removed. Now the Lord Christ is the Spirit of JHWH, and where the Spirit of JHWH is, there is freedom. And all of us, with unveiled faces, seeing the glory of JHWH as though reflected in a mirror, are being transformed into the same image from one degree of glory to another, for this comes from JHWH, the Spirit.
According to this reconstruction, idenitifying Christ as the spirit of JHWH establishes the connection between JHWH and Christ. Turning to JHWH in the Old Testament is the samea as turning to Christ now. Because the names of God are represented by nomina sacra, the readers of the Canonical edition arrive at the same conclusion faster. For them 2 Cor 3:16 means that as the Jews turn to the Lord (Jesus), the veil is removed from their reading of the Old Testament.
The editors did not mind this misrepresentation of Paul. The effect on Christian readers- that Jesus and JHWH become synonyms- was probably intended." (First Edition of the New Testament pg. 67)
There are arguments against this, but it is not a weak point to consider.