Bonjour (et bonne année, dirait-on...) !
Believe it or not, I saw your post only today, 20 months after it was posted !
Regarding the translation :
could we say "ni masculin ni féminin" (we could add "sexe" as well but would it not be to long ?), or "ni virilité, ni féminité" ?
Regarding the interpretation :
In my mind, Paul mentions before all what should be very vivid for the reader. On the other hand, the "gaps" he mentions are increasingly deep.
Edit :
Warning : distorted memories
It reminds me a text where a Greek (Xenophanes ?) says that he thanks the gods because 1) he is a man and no woman 2) he is a freeman, not a slave and 3) he is a Greek, not a Barbarian.

The text I was thinking about is really the following (from Diogenes Laertius, I, 33-34 [Life of Thales]) :
"But Hermippus, in his Lives, refers to Thales what has been by some people reported of Socrates; for he recites that he used to say that he thanked fortune for three things: first of all, that he had been born a man and not a beast; secondly, that he was a man and not a woman; and thirdly, that he was a Greek and not a barbarian."
(Translated by C.D. Yonge, see
here)
Greek text :
[face=SPIonic]
(/Ermippoj d’ e)n toi=j Bi/oij ei)j tou=ton a)nafe/rei to\ lego/menon u(po/ tinwn peri\ Swkra/touj. e)/faske ga/r, fasi, triw=n tou/twn e(/neka xa/rin e)/xein th=| Tu/xh| : Prw=ton me\n o(/ti a)/nqrwpoj e)geno/mhn kai\ ou) qhri/on, ei)=ta o(/ti a)nh\r kai\ ou) gunh/, tri/ton o(/ti (/Ellhn kai\ ou) ba/rbaroj. [/face]
This is not exactly parallel to Paul's text
Now, legalistic or ontological ? I don't believe Paul had this distinction in mind, but I would say rather ontological. You know, slaves were seen as "inferior" beings, with a bad, weak nature. Of course, this completely illusory opinion helped the people to legitimate slavery.
Hoping you are still around...
Encore toutes mes excuses.