This is the relevant section of Smyth:
- When a verb of denying, refusing, hindering, forbidding, etc., is itself negatived, either directly or by appearing in a question expecting a negative answer, the infinitive has μὴ οὐ. Here both the introductory clause and the dependent clause have virtually an affirmative sense.
οὐδεὶς πώποτ᾽ ἀντεῖπεν μὴ οὐ καλῶς ἔχειν αὐτούς (τοὺς νόμους) no one ever denied that they (the laws) were excellent D. 24.24, τίνα οἴει ἀπαρνήσεσθαι μὴ οὐχὶ καὶ αὐτὸν ἐπίστασθαι τὰ δίκαια; who, think you, will deny that he too understands what is just? P. G. 461c ( = οὐδεὶς ἀπαρνήσεται). But μὴ οὐ is not used after οὔ φημι, οὐκ ἐῶ, οὐκ ἐθέλω (2692 a).
a. μὴ οὐ with the infinitive here, and elsewhere, is used only when the introductory word or words has an actual or a virtual negative. Since, in ἀρνοῦμαι μὴ ταῦτα δοᾶσαι I deny that I did this, μή confirms the negative idea in ἀρνοῦμαι, so in οὐκ ἀρνοῦμαι μὴ οὐ ταῦτα δρᾶσαι I do not deny that I did this, οὐ after the strengthening μή confirms the οὐ prefixed to the leading verb. Cp. “Je ne nie pas que je ne sois infiniment flatté” (Voltaire). In the first sentence μή repeats the ‘negative result’ of ἀρνοῦμαι (single sympathetic negative, untranslatable); in the second sentence οὐ is repeated with the infinitive to sum up the effect of οὐκ ἀρνοῦμαι (double sympathetic negative; both untranslatable). After verbs negative in meaning (deny, etc.) μή and μὴ οὐ cannot be translated in modern English (see 2739). After verbs not negative in character but preceded by a negative, and after virtually negative expressions, μή or μὴ οὐ has a negative force (2745, 2746).
b. μὴ οὐ with the infinitive regularly indicates a certain pressure of interest on the part of the person involved.
http://www.perseus.tufts.edu/hopper/text?doc=Smyth+grammar+2742&fromdoc=Perseus%3Atext%3A1999.04.0007
κωλύω can take either simple acc + inf or acc + inf with redundant μή/μή οὐ. The latter is apparently rarer in Attic prose (which fact is frustratingly missing from Smyth and some others, like Sidgwick and Nash-Williams; not Dickey though).
LSJ asserts that acc + inf with redundant μή is rare in Attic prose, and I think that’s the source of Dickey’s assertion to that effect. Where did that assertion come from? Was there a statistical study of Attic prose usage on this point? German scholars in the 19th century did a lot of painstaking work compiling all extant instances of specific constructions – that’s the source of the information in Smyth and Goodwin – so perhaps that’s where this assertion came from. But of course the extant body of Greek prose is far less than what existed in antiquity, so statistical studies of specific, not very common usages must be somewhat hit or miss,
However, in the sentence you asked about, μὴ κωλύοιεν is negative, and that’s where μή οὐ is used. Even there, it seems to be an alternative to simple μὴ. So perhaps you and Nash-Williams are both right.
Goodwin, Greek Moods and Tenses 817 provides a thorough review of these constructions (the clearest treatment, I think; I’ve edited out some things and bolded others, but you’ll want to take a look at the entire discussion):
After verbs and other expressions which denote hindrance or freedom from anything, two forms are allowed, the simple infinitive, and the genitive of the infinitive with τοῦ.
Thus we can say (a) εἴργει σε τοῦτο ποιεῖν (747) and (b) εἴργει σε τοῦ τοῦτο ποιεῖν (798), both with the same meaning, he prevents you from doing this. As the infinitive, after verbs implying a negation, can take μή to strengthen the previous negation without otherwise affecting the sense (815, 1), we have a third and a fourth form, still with the same meaning: > (c) εἴργει σε μὴ τοῦτο ποιεῖν, > and (d) εἴργει σε τοῦ μὴ τοῦτο ποιεῖν, he prevents you from doing this. . . .
If the leading verb is itself negatived (or is interrogative with a negative implied), the double negative μὴ οὐ is generally used instead of μή in the form (c) [with the simple infinitive, > but probably never in the form (d) with the genitive of the infinitive; as οὐκ εἴργει σε μὴ οὐ τοῦτο ποιεῖν, he does not prevent you from doing this (815, 2), but not τοῦ μὴ οὐ τοῦτο ποιεῖν. (See also 811, for τὸ μὴ οὐ.) E.g.
a) Κακὸν δὲ ποῖον εἶργε τοῦτ᾽ ἐξειδέναι; SOPH. O.T. 129. Παιδὸς Φέρητος, ὃν θανεῖν ἐρρυσάμην. EUR. Alc. 11. Ἐπὶ Ὀλύνθου ἀποπέμπουσιν, ὅπως εἴργωσι τοὺς ἐκεῖθεν ἐπιβοηθεῖν. THUC. i. 62. Ἄλλως δέ πως πορίζεσθαι τὰ ἐπιτήδεια ὅρκους ἤδη κατέχοντας ἡμᾶς (ᾔδειν). XEN. An. iii. 1, 20. Εὐδοκιμεῖν ἐμποδὼν σφίσιν εἶναι. PLAT. Euthyd. 305 D. Εἰ τοῦτό τις εἴργει δρᾶν ὄκνος, “if any hesitation prevents you from doing this.” Soph. 242 A. Τὴν ἰδέαν τῆς γῆς οὐδέν με κωλύει λέγειν. Plat. Phaed. 108 D. Τὸν Φίλιππον παρελθεῖν οὐκ ἐδύναντο κωλῦσαι. DEM. v. 20.
(b) Τοῦ δὲ δραπετεύειν δεσμοῖς ἀπείργουσι; XEN. Mem. ii. 1, 16. Τὸ γὰρ ψευδόμενον φαίνεσθαι καὶ τοῦ συγγνώμης τινὸς τυγχάνειν ἐμποδὼν μάλιστα ἀνθρώποις γίγνεται. Id. Cyr. iii. 1, Id. Cyr. 9. Εἶπεν ὅτι κωλύσειε (ἂν) τοῦ καίειν ἐπιόντας. Id. An. i. 6, Id. An. 2. Ἐπέσχομεν τοῦ δακρύειν. PLAT. Phaed. 117 E (cf. 117 C, quoted in 811). Ἀπεσχόμην τοῦ λαβεῖν τοῦ δικαίου ἕνεκα. DEM. xix. 223.
(c) “Θνητούς γ᾽ ἔπαυσα μὴ προσδέρκεσθαι μόρον” AESCH. Prom. 248. “Τοὐμὸν φυλάξει σ᾽ ὄνομα μὴ πάσχειν κακῶς” SOPH. O.C. 667. Ὅπερ ἔσχε μὴ τὴν Πελοπόννησον πορθεῖν, “which prevented him from ravaging the Peloponnesus.” THUC. i. 73. Διεκώλυσε μὴ διαφθεῖραι. Id. iii. 49. Ἐπεγένετο κωλύματα μὴ αὐξηθῆναι. Id. i. 16. Πέμπουσι κήρυκα, ὑποδεξάμενοι σχήσειν τὸν Σπαρτιήτην μὴ ἐξιέναι. HDT. ix. 12. “Εἶργε μὴ βλαστάνειν.” PLAT. Phaedr. 251 B.
Οὐ γὰρ ἔστι Ἕλλησι οὐδεμία ἔκδυσις μὴ οὐ δόντας λόγον εἶναι σοὺς δούλους. HDT. viii. 100. (See 815, HDT. 2; 816.) Οὐ δυνατοὶ αὐτὴν ἴσχειν εἰσὶ Ἀργεῖοι μὴ οὐκ ἐξιέναι. Id. ix. 12. “Ὥστε ξένον γ᾽ ἂν οὐδέν᾽ ὄνθ᾽, ὥσπερ σὺ νῦν, ὑπεκτραποίμην μὴ οὐ συνεκσῴζειν.” SOPH. O.C. 565. Τί ἐμποδὼν μὴ οὐχὶ ὑβριζομένους ἀποθανεῖν; XEN. An. iii. 1, 13. (Τί ἐμποδών implies οὐδὲν ἐμποδών.) Τίνος ἂν δέοιο μὴ οὐχὶ πάμπαν εὐδαίμων εἶναι; “ what would hinder you from being perfectly happy?” Id. Hell. iv. 1, Id. Hell. 36.
http://www.perseus.tufts.edu/hopper/text?doc=Goodwin+syntax+807&fromdoc=Perseus%3Atext%3A1999.04.0065
Thanks for bringing this up. It forced me to review a point of Greek syntax that had been lost to me in the fog of time.