The word ουκουν is tricky. It can be either οὐκοῦν or οὔκουν. Plato’s original text of course was not marked with accents or other diacritics.
Here is the relevant part of the LSJ entry for οὐκοῦν:
A.like οὔκουν, of οὐκ and οὖν, but differing in meaning and accent, cf. A.D. Conj.257.18sqq., Hdn.Gr.1.516, Phryn.PSp.98B.
I. in questions, inviting assent to an inference, or to an addition to what has already received assent, οὐκοῦν δοκεῖ σοι . . ; you think then, do you not, that . . ? X.Cyr.2.4.15, Mem.1.4.5, cf. 4.2.20, Pl.Prt.332b, 360b-d, Cra.416c, etc.: with hortatory subj., οὐκοῦν καὶ ἄλλους σε φῶμεν δυνατὸν εἶναι ποιεῖν (sc. ῥήτορας ἀγαθούς); Id.Grg.449b: folld. by οὐ when a neg. answer is invited, “οὐκοῦν οὐκ ἂν εἴη τὸ μὴ λυπεῖσθαί ποτε ταὐτὸν τῷ χαίρειν” ; Id.Phlb.43d, cf. Phd. 105e; “οὐκοῦν οὐδ᾽ ἂν εἷς ἀντείποι” ; D.16.4.
II. in affirm. sentences, surely then, “οὐκοῦν, εἰ ταῦτα ἀληθῆ, πολλὴ ἐλπὶς ἀφικομένῳ οἷ ἐγὼ πορεύομαι” Pl.Phd.67b: with subj. or imper., οὐκοῦν διδάσκωμεν αὐτόν, ἀλλὰ μὴ λοιδορῶμεν let us teach him, then, Id.La.195a; οὐκοῦν . . ἱκανῶς ἐχέτω let this then suffice, Id.Phdr.274b, cf. 278b, Luc.DMort.23.3; “οὐκοῦν ἂν ἤδη . . λέγοι” Ar.Pax43: with a prohibition, “οὐκοῦν μὴ . . αὐτομολήσῃς” Aeschin.1.159; οὐκοῦν ὑπόλοιπον δουλεύειν slavery, then, is the only alternative, D.8.59.
οὔκουν, with the accent on οὔκ-, is negative, but it also can introduce a question expecting an affirmative answer: “Isn’t it true that . . .?” LSJ οὔκουν:
II. in impassioned questions, almost = οὐ alone, οὔκουν, Προμηθεῦ, τοῦτο γιγνώσκεις, ὅτι . . ; A.Pr.379, cf. Eu.725; “οὔκουν ἐγώ σοι ταῦτα προύλεγον πάλαι” ; S.OT973; “οὔκουν τάδ᾽, ὦ παῖ, δεινά” ; Id.Ph.628, cf. E.IT 1190, 1196, Ar.Eq.820, Lys.10.12,13, Is.5.34, 11.13, Aeschin.1.85, 2.87, al. (sts., but prob. wrongly, written οὐκοῦν or οὐκ οὖν): sts. separately, οὐ δεινὸν οὖν δῆτ᾽ . . ; Ar.Eq.875: freq. with 2sg. fut., to express an urgent or impatient imper., “οὔκουν ἐπείξῃ τῷδε δεσμὰ περιβαλεῖν” ; A.Pr.52; “οὔκουν μ᾽ ἐάσεις κἀκτὸς εἶ” ; S.OT676, cf. Ant.244, Ar.Ra.200, Pl.71, Pl.Smp.175a: also with τις and 3sg. fut., οὔκουν τις ὡς τάχιστα . . ἀναγκάσει . . ; S.OC897; or opt. with ἄν, οὔκουν ἂν εἴποις . . ; Id.Aj.1051: with neg. repeated, “οὔκουν ἐάσεις οὐδ᾽ ὑπ᾽ εὐφήμου βοῆς θῦσαί με” ; Id.El.630.
So the sentence in Phaedrus can be either a question inviting assent or something like “surely then”, depending on the modern translator’s judgment. It’s probably not an “impassioned” question, so οὐκοῦν seems right here, but that doesn’t answer the question of whether the μεν clause is a statement or a question.
Apparently Hackforth and Waterfield take the μεν clause as a question, but Hackforth takes the δε clause as a statement, while Waterford takes it as a question. But with the continuation of the sentence after Phaedrus’ interruption with the δε clause, I think I’d take the μεν clause as a statement. In the Oxford edition, Burnet takes both μεν and δε as statements, and so does Harvey Yunis in the Cambridge Greek and Latin Classics series. What does Robin do?
There’s no way to know whether Plato intended it as a question. This is a persistent issue in Plato, especially, almost every time that ουκουν introduces a sentence. But fortunately, in the context of 274b (and in most other occurrences, too), it really doesn’t make even a slight difference.
I guess it could conceivably be οὔκουν as a negative statement and not as a question, instead of οὐκοῦν. Then Socrates would be saying, as you originally suggested, that the topic of τὸ τέχνης τε καὶ ἀτεχνίας λόγων πέρι is incomplete, implying that τὸ εὐπρεπείας δὴ γραφῆς πέρι καὶ ἀπρεπείας, πῇ γιγνόμενον καλῶς ἂν ἔχοι καὶ ὅπῃ ἀπρεπῶς is part of the first topic and remains to be addressed. But again, μεν . . . δε signals that the second topic is something new, contrasting with the first topic, not a part of it. (And I believe this is consistent with the general thrust of Phaedrus, drawing a distinction between the substance of a discussion and the inadequacy of what can be put into writing.) So I don’t think there’s much question that οὐκοῦν should be read here.