At any case, assuming for mere sake of discussion the authenticity of the relatives passages (basically Romans 1:3 and Galatians 4:4), my memory goes to this McGrath's observation about paradigmatic parallelism.
There is, however, a common element of ancient thought which has important implications for the understanding of Ascension of Isaiah. In 7:10 we read, “And as above, so also on earth, for the likeness of what (is) in the firmament is here on earth.” As Carrier notes, “the narrative goes out of its way to explain that the firmament contains copies of everything on earth.” And yet, presumably because of his aim to present a case for mythicism, Carrier does not discuss the natural implication of this: that even if the celestial Beloved only descended as far as the firmament, and was crucified there by demons, this would mirror some corresponding occurrence on earth. This is reminiscent of what we find depicted or hinted at in a number of Docetic texts. While the earthly Jesus is crucified, the real Jesus is seen above the cross, a spiritual being whom they cannot harm, laughing at the fools who think they have genuinely crucified him.
(my bold)
Naturally if something happened in heaven for Jesus, then this would mirror some corresponding occurrence on earth.
Paul talks already about a "body of Christ" on the earth: the totality of the Pauline Christians. What need of an earthly Jesus distinct from this body ?
Therefore McGrath's argument fails by his own logic.
So Earl Doherty:
One may ask why it is that Paul bothers to say that Christ was born of a woman, since this should be an obvious biological fact to his readers. His point may be that he wishes to stress the paradigmatic parallel between believers — who are themselves born of woman, as well as born under the law of the old covenant which Paul wants to abrogate — and Christ himself. Only through counterpart characteristics can paradigmatic effects exists. But such relationships by definition operate between higher and lower worlds, between the spiritual and the material. It follows, then, that Christ and his features must belong to the higher world, in order to be in appropriate counterpart to those of Paul's readers.
(The Jesus Puzzle, p. 124-125, my bold)