Jesus is the firstborn of Mary: it is customary in the Scriptures to call not the one who is followed by siblings, but the one who is born first, the firstborn. Thus, Paul also calls Christ the firstborn Son of the Father (Heb 1:14; cf. Ex 34:19 Num 18:15). Among the Jews, the firstborn is primarily a legal concept; therefore, it also applies to the only child. Firstborn can mean the same as the only child, as Christ is called in John (1:14), because according to the scriptural language usage, the only children are also called firstborns. See Joshua 17:1. According to the Old Testament legal conception, the firstborn, as the future head of the family, has a distinguished position in the family; he receives a larger portion of the inheritance than the other children.
Romans 8:29: it's not genetivie what is used, but look after the "ek" in the Greek text, and of course here Paul is talking about Jesus status according to his Incarnation, when He became human as well, talking about the content and nature of adopted sonship of the saved ones, so it is about Jesus coming into the world so that we too may become children of God. The adoption of a son is the adoption of a foreign person for sonship and inheritance. By grace, God not only adopts a stranger (that is, not born from his reality), but a disloyal, hostile, sinful person; and not just in a moral and legal sense, as it happens among people, where adoption does not give the adoptee any inner value; but in a majestic and mysterious way, it compensates for natural birth, as it grafts the seed of divinity into him, making him not only morally, but physically partake in the divine nature, thus imitating the superiority of natural sonship, the community of nature (so to speak, the blood), so that we are now not only in name, but in reality, the sons of God. As a result, our adopted sonship is a copy, a faithful imprint of the eternal filiation of God. At the same time, we have become brothers of Christ in a very special sense; and it is understandable why it is customary to attribute son-ship adoption to the Father. Jesus Christ, who on the one hand as a man of the same nature as us and the firstborn among the brothers is one of us, on the other hand he is completely pure and holy.
Colossians 1:18 doesn't mean he's the first person to ever die. It means that by His resurrection, He overcame death and made is possible for a new generation of humans called to glorious resurrection (Rom. 8:29). And He took over the power and the primacy over everything.
Colossians 1:15: the apostle intends to express the preeminence of the Firstborn above of the whole creatured world: Christ existed before the entire created world and stands above it. He is the only-begotten before all creatures, and He Himself is not a creature; for He is the cause of existence for all creatures, and therefore cannot be a creature, as it becomes clear from the continuation of the text: He is the Firstborn of the whole creation BECAUSE in Him all things were created. He is heir of all things, and inherits the throne of his ancestor David. By the way if apply this to the Messianic Kingship of Christ, it's meant according to his Incarnate human natura, according to which, He is a creature, even for the Orthodoxy. This interpretation, as we can see, is also compatible with the interpretation of the Watchtower, apparently they only fight because they lose the very few "proof texts".
Athanasius wrote:
Not then because he was from the Father was he called “Firstborn,” but because in him the creation came to be; and as before the creation he was the Son, through whom was the creation, so also before he was called the Firstborn of the whole creation, the Word himself was with God and the Word was God. … If then the Word also were one of the creatures, Scripture would have said of him also that he was Firstborn of other creatures; but in fact, the saints’ saying that he is “Firstborn of the whole creation” demonstrates that the Son of God is other than the whole creation and not a creature. (Discourses Against the Arians II.63)
Ambrose similarly writes:
The apostle says that Christ is the image of the Father—for he calls him the image of the invisible God, the firstborn of all creation. Firstborn, mark you, not first created, in order that he may be believed to be both begotten, in virtue of his nature, and the first in virtue of his eternity. (Of the Christian Faith I.VII.48)
John Chyrsostom also wrote a long dissertation on this theme in his 3rd Homily on Colossians.
And you misinterpret Origen many times, so it seems I am talking to the wall.