I'm reading St Athanasius and I don't get the feeling he is explicitly rejecting the literal reading of Genesis: I think you're reading that into him. It seems rather he is assuming it and using it as the basis for his allegorical interpretation, just as St Gregory the Theologian did. Plus there are many other Fathers where the literal interpretation is much more prominent. For instance, St Athanasius certainly speaks of our natural state as corruptible. But it seems to me he is using "natural" in one particular sense, to mean nature without grace, after the Fall, which is corruptible. Other Fathers, however, speak of our natural state as incorruptible, e.g. Abba Dorotheus in his First Instruction:
In the beginning, when God created man (Gn. 2:20), He placed him in Paradise and adorned him with every virtue, giving him the commandment not to taste of the tree which was in the midst of Paradise. And thus he remained there in the enjoyment of Paradise; in prayer, in vision, in every glory and honor, having sound senses and being in the same natural condition in which he was created. For God created man according to His own image, that is, immortal, master of himself, and adorned with every virtue. But when he transgressed the commandment, eating the fruit of the tree of which God had commanded him not to taste, then he was banished from Paradise (Gn. 3), fell away from the natural condition, and fell into a condition against nature, and then he remained in sin, in love of glory, in love for the enjoyments of this age and of other passions, and he was mastered by them, for he became himself their slave through the transgression.
So in one sense our current corruption is natural, but in another sense it is unnatural. It doesn't seem to me reasonable to use St Athanasius' words to bolster evolutionist claims that corruption and death are natural, when other saints say the opposite about our nature. The Fathers who speak of our condition becoming unnatural clearly consider our prelapsarian condition, the condition to which we were restored by Christ, to be the only truly natural one.
Also, I'm not so sure it's correct to say that human nature is defined as a union of human and angelic, in the way Christ is a union of divine and human natures. It seems rather that human nature is its own thing, while it shares characteristics with both the animals and the angels. Fr Seraphim actually stresses this in his essay.
Finally, your claims about the purposes of the Genesis creation "myth", that they were only intended to "delegitimize" other pagan myths, are interesting and intellectually attractive, but they are not patristic. They are what a secular historian of comparative religion might claim, but not what traditional Orthodox exegesis claims.
In conclusion, my impression is that you overly rely on one single Father, St Athanasius, to the exclusion of other Fathers, whose words cannot be so molded to fit your particular theistic evolutionism. Furthermore, you insist on a certain interpretation of St Athanasius' words to convey your idea that man was corruptible at first, being descended from corruptible animals by natural generation, was lifted from corruption and placed in Paradise, and then fell away to his original state. However, St Athanasius does not say this in so many words; only his particular use of the term "nature" can be made to fit that interpretation. Moreover, the witness of other Fathers makes it clear that the Church has traditionally held man to have been originally created in incorruption, and that Man's current state of corruption is solely due to his fall, and not to do with nature as it was before the fall. As a believer in the consensus patrum
, I would prefer to assume that St Athanasius believed the same things as the other Fathers about the nature of man. If in fact he did not, then I would still have to say that his views do not accord with the patristic consensus.