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In Genesis 4:15, as you indicate, J_Edwards, “the Lord appointed a sign for Cain”, saying to him, “whoever kills Cain, vengeance will be taken on him sevenfold.”. It doesn’t say here, and I can’t find anywhere else in Scripture where it says, whether this sevenfold vengeance will be executed directly by God, or by way of human free-agency. If it’s by way of human free agency, then this sevenfold vengeance is certainly a form of positive law. But since I see no clear indication that the sevenfold vengeance is to be executed via human free agency, I see no reason to assume that it’s positive law.
Genesis 4:14-15 "Behold, Thou hast driven me this day from the face of the ground; and from Thy face I shall be hidden, and I shall be a vagrant and a wanderer on the earth, and it will come about that whoever finds me will kill me." So the LORD said to him, "Therefore whoever kills Cain, vengeance will be taken on him sevenfold." And the LORD appointed a sign for Cain, lest anyone finding him should slay him.

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The same basic argument applies to Adam. I agree that Adam was “the covenant head”. But, “in the day that you eat from it you will surely die” (Gen. 2:17), doesn’t clearly indicate who will execute the penalty of death. I suspect that most people reading this believe that death will come on these people because God’s wrath will come on these people. Even though I haven’t read all of it, I suspect that if I claimed that virtually all historically reliable covenant theology posits that the penalty of death comes on these people by way of God’s wrath, not by way of human free agency, virtually no one would argue. So the lack of human free agency in the execution of the penalty in Genesis 2:16-17 means that this is also an unlikely instance of God-mandated positive law.
While I would agree that death of any type is an appointment designed by God (Heb. 9:27), He still uses secondary causes to execute it (i.e. whether it be nature, angels (Gen 3:24) or man, (Rom. 13:4-5), etc.). And since God knew/foreordained the "secondary causes" prior to the fall they are included in His paradigm for death!!! This is seen working itself out in chapter 4, where if you study Cain's response closely you will see that he did not necessarily fear God, but man's retaliation!

The Ten Commandments were in force before Moses. Briefly (very briefly) just how many of the commandments were broken in the Garden? Eve choose to believe Satan—she had another god before God (2 Cor 4:3-4; Ex 20:2-3). Part and parcel of the 2nd commandment includes “who love Me and keep My commandments” (Ex 20:4-6) which both Adam and Eve violated. Eve believed the Lord’s name was vain (Ex 20:7) –“Hath God said.” Adam and Eve dishonored their only Parent (Ex 20:12). The first murder of course occurred in the Garden of Eden (Ex 20:13). Didn’t Adam and Eve commit spiritual adultery—were they not married to God (Ex 20:14; the bride of Christ)? The first theft was in the Garden (Ex 20:15; Gen 3:11) – the fruit. Satan lied to the woman and then the woman lied to herself (Ex 20:16; Gen 3:1-4). And of course there was coveting (Ex 20:17; Gen 3:6). Now the only one I have left out is keeping the Sabbath—but in their sin they could no longer keep the Sabbath—which is communion with God—instead they hid (Gen 3:10)!!! Wasn’t the whole of God’s command to Adam to love Him (2 Kings 23:25; Matthew 22:36-40; Galatians 5:14) and not the tree of KGE? Without obedience to commandments there can be no relationship with God!

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1 John 3:11-12 For this is the message which ye heard from the beginning, that we should love one another: not as Cain was of the evil one, and slew his brother. And wherefore slew he him? Because his works were evil, and his brother's righteous.

3:24 And he that keepeth his commandments abideth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he gave us.

4:21 And this commandment have we from him, that he who loveth God love his brother also.

5:2-3 Hereby we know that we love the children of God, when we love God and do his commandments. For this is the love of God, that we keep his commandments: and his commandments are not grievous.
If Adam and Eve had a relationship with God prior to the fall, then there was commands prior to the fall ("don't eat" is a command. A positive command that Adam as covenant head was suppose to obey and enforce). However, Genesis 3 is more than just about eating some forbidden fruit, it has to do with the motives behind the eating and those motives violated the 10 commandments and thus, Adam and Eve's (not to mention the human races) relationship with God, et. al. To say there was no positive law before the fall is to say: (1) there was no real relationship between Adam/Eve & God, and (2) that Adam had no way for obeying and enforcing the command not to eat the forbidden fruit, which in turn makes the whole of creation/fall a sham!

Reformed and Always Reforming,