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 :
[size=134](/ερμιππος δ’ ἐν τοῖς βίοις εἰς τοῦτον ἀναφέρει τὸ λεγόμενον ὑπό τινων περὶ σωκράτους. ἔφασκε γάρ, φασι, τριῶν τούτων ἕνεκα χάριν ἔχειν τῇ τύχῃ · πρῶτον μὲν ὅτι ἄνθρωπος ἐγενόμην καὶ οὐ θηρίον, εἶτα ὅτι ἀνὴρ καὶ οὐ γυνή, τρίτον ὅτι (/ελλην καὶ οὐ βάρβαρος. [/size]
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.