I commend your question in explicitly stating your assumptions, such that the question follows.
One of your assumptions is that
the default state is that all women (married and not, young and not) are obligated to cover their hair (as שער באשה ערוה - Brochos 24a, ...)
(The ellipses are my own; because I only wish to discuss the gemara in Berachot, rather than the Shulchan Aruch's understanding of a different gemara.) The question is whether the words באשה in the gemara you mentioned refers to all women, married or not, and young or not.
It is not obvious because the word אשה in Hebrew sometimes refers to a woman, and sometimes it refers to a wife (an אשת איש). If in this gemara it refers specifically to a married woman, then this particular assumption upon which your question rests does not hold.
If we look at Rashi in the context of this gemara, he writes about shok be'isha erva:
שוק - באשת איש:
ערוה - להסתכל וכן באשתו לק"ש:
He writes that it applies to a married woman specifically. It stands to reason that this definition of specifically eshet ish also applies for the next statement in the gemara as well.
In terms of your assumption on the second source, that Ketubot 72 (which you should cite rather than Shulchan Aruch's interpretation of it), which speaks to bnot Yisrael, it is a question of balancing ambiguous sources. The context of the gemara in Ketubot speaks of women leaving marriage without their ketubah, so is explicitly married women. The Tanna deVei Rabbi Yishmael finds (as the other answer notes) this warning to women from the context of Sotah, so even though employs the term bnot Yisrael, the context might mean Jewish daughters in the sense of married Jewish daughters. It does not explicitly say non-married Jewish daughters.
Then, as the Chelkas Mekokek points out, there are other sources such as an explicit Mishna in the second perek of Ketubot that יוצאה בהינומא וראשה פרועה is evidence that she was a virgin. As a result, this idea that unmarried women must cover is only to non-virgins. That is one way of harmonizing and balancing the competing and ultimately ambiguous sources.