AOA and I had gotten your first point. I never argued that the Qur'an says circumcision is prohibited, what I said was that the Qur'an doesn't indirectly prescribe or implicitly sanction (between which I see no distinction frankly) circumcision. And I understand that several concepts in the Qur'an, Saum for instance, aren't defined in detail because their meaning was already well known to its original recipients. But this doesn't imply or say anything about us recognizing the arbitrary day to day practices of a prophet as some how sanctioned by the Qur'an as divine and a part of Islam. That the recipients of the Qur'an understood what the Milat of Ibrahim meant could just as easily, and more sensibly, imply that this Milat refers to the Qur'an itself, as it is the book conveying the last message from God to humanity, the Deen of humanity. What could better convey this Deen of Ibrahim?
This idea that because certain practices were so widely practiced around the time a book is revealed that they should be deemed divinely sanctioned since a prophet incidentally also took part is precisely what is so dangerous here. So when you ask "If you think my argument can be applied to “retro-grade practices” related to Creed of Ibrahim, please bring one such example," it is easy to come up with several: slavery and subjugation of women are obvious examples. Applying your dangerous logic to these, and mixing in some traditional interpretations of the Qur'an a la "what the right hand possesses," and you could easily give divine sanction to both these evils.