First, in the commandment of Torah study the distinction is different. The commandment of Torah study is reciting the Shema morning and evening. Torah study by virtue of its fundamental value is everything else. This is not a relationship between command and content, but between command and that which goes beyond the command. But regarding commandments in general, there is indeed the command and the content/essence.
As for your question, rabbinic laws too contain compelling reasonings, but they are weaker. For example, separating food from waste, which is prohibited only rabbinically, has a compelling logic for forbidding it. But this is a selection not strong enough for the Torah to forbid it. The underlying logic, however, is God's logic, just as in separating waste from food. It is just that even according to His standard, it is not strong enough to forbid. Note well: when I say the reasoning is not strong, I do not mean it is not correct. It is entirely correct, but what the reasoning says is that it is fitting to forbid, while the prohibition is weak. For example, the Torah could have raised the threshold of prohibition and forbidden only things punishable by karet or execution by a religious court, and then all ordinary prohibitions would have been permitted by Torah law. But that would not mean there is no compelling reasoning in a regular prohibition. The reasoning is compelling, but the prohibition that emerges from it is weaker. And so too in our case, when the threshold is above rabbinic prohibitions.
Think about it: after all, even compelling reasoning is the reasoning of the person who thinks it. So why are you satisfied with compelling reasoning? Clearly because compelling reasoning expresses the will of the Holy One, blessed be He. Rabbinic reasoning also expresses His will, only it is a weaker will. One can say that a rabbinic prohibition is half a Torah prohibition—and not as people usually think, that there is no Torah-level problem in it at all.
From this you can also understand that if there is non-compelling reasoning—not in content, but in its logical force—that is not a prohibition at all, but at most a possible prohibition.
And as is known, the later authorities already distinguished between two types of rabbinic restrictions: expansion of the forbidden labor rabbinically—these are exactly the weak reasonings I defined above—and a decree lest one come to the labor itself, where there is no essential reasoning at all, only a technical reasoning that perhaps one may fail and come to a Torah prohibition. But even that is a compelling reasoning; it is just that the act in itself is not problematic at all. I should note, however, that Rabbi Abraham Isaac Kook wrote in Mussar Avikha that even fences and decrees contain something of the prohibition itself. And the words of the Netivot HaMishpat, that rabbinic prohibitions are on the person and not on the object, were said either about prohibitions of the second type, or perhaps also the first type, because at the end of the day the Torah did not forbid it. The problem is in the object—a problem of half a Torah prohibition—but the prohibition is only on the person.
See an interesting article about this by Daniel Weil in Higayon 1. I do not agree with his bringing quantum logic into the sages, but his proof from the sugya of the golden city that a rabbinic prohibition is half a Torah prohibition is good.

It follows that when one deals with a rabbinic law—at least those that are half a Torah prohibition—one is learning God's will, but the weaker wills. As for fences, there is room to discuss, because in themselves they involve no problem at all. But even person-based reasonings are God's will of "make a safeguard for My safeguard."

If you study what the nations established and find in it reasoning, you have learned Torah. Every correct reasoning contains an aspect of Torah study. True, so long as the sages did not insert it into Jewish law as a rabbinic law, perhaps it is not really Torah in the object. In any case, it is Torah not because a non-Jew said it, but because it is true.