Remember I am not the one who brought forth Romans 2:15.
When I went to see exactly what Romans 2:15 said I found it to be in the () starting in verse 13. And further found that verse 11 states For there is NO respect of persons in God.
This is contrary to what all denominations proclaim.
You seem to think that the reference to the day of the Lord or God's not being a respecter of persons has some telling or determining bearing on whether the text can rightly be understood as supporting the idea that even those without the so-called positive law still have some, possibly innate, understanding of right and wrong.
I'm not trying to score points. I'm trying to understand your contention and I'm failing miserably.
I see no parentheses in verse 13. When Paul says God is no respecter of persons (it looks to me something like "For there is no favoritism by God"), in the context I see him explaining that therer will be tough times for whoever does evil, Jew or Greek, and glory, honor, and peace for whoever does good, Jew or Greek. And then there is verse 11 with its postpositive γαρ: ου γαρ εστιν προσωπολημψια παρα τω θεω.
So it seems that the aspect of persons which God does not consider is whether they are Jew or Greek. The γαρ (= "for", more or less) serves to join the statement of principle (no regard for persons) to the instance (being a Jew or a Greek won't make any difference about the kind of consequences deeds incur.)
Verse 12 continues and restates the idea that sinners (οσοι ... ημαρτον) without the law (ανομως) or "in" the law (εν νομω) will be judged, without or within the law.
13 contrasts hearing and doing - doing justifies, hearing doesn't.
14 ... and I believe this is where the nub of the disagreement is, says (and again there is the postpositive γαρ) FOR, when the ethne not having the law [nonetheless] DO "by nature" (φυσει) what the law requires, they are a law to themselves [!].
Then (and I take this as an explanation of the idea of being a law to oneself, an expression which could stand some explication) Paul says that what the law requires is γραπτον εν ταις καρδιας -- written in their hearts.
So that seems to me to support the idea that at least some aspects of the moral law are revealed to all.
And I don't remember now but I think that was papertyger's initial contention, and it sure seems to me that walking through these verses supports that contention.
IN general, I think Augustine is right. We are born with a lack in us which can be filled only by God. One aspect of that lack is a moral sense, and, as it were, seeing the shape of the hole left in our soul, we can begin to understand the shape of the thing which would fill it. And the moral aspect of that understanding is sorta kinda maybe like what theologians mean by "natural law".
IMHO, somebody needs to write a paper on Paul's use of φυσις and its cognates.