I'm not sure I follow you - are you suggesting that in v. 14 there is a break from the third person pronouns in the preceding verses, and that the prophecy is to/concerning the elect?
I've done my best to assemble all the uses of propheteuo in the LXX, Apocrypha and NT which include an indirect object. I've tried to classify them according to the constructive (preposition or dative) and determine whether they follow the alleged special idiom. Please have a look if you would, and let me know if you think it would be far-fetched to suggest the existence of such a special idiom, or it remains far-fetched:
Propheteuo + dative used of prophecy which was deemed (either by the writer or the speaker in the narrative) as inauthentic:
Propheteuo + pronoun + genitive used of prophecy which either the writer or the speaker in the narrative believes to be authentic:
"...ὅτι ὁράσεις ψευδεῖς καὶ μαντείας καὶ οἰωνίσματα καὶ προαιρέσεις καρδίας αὐτῶν αὐτοὶ προφητεύουσιν ὑμῖν" (Jer. 14:14 LXX; cp. 14:16 which has similar construction with third person pronoun)
"καὶ ἐν Βαβυλῶνι ἀποθανῇ καὶ ἐκεῖ ταφήσῃ, σὺ καὶ πάντες οἱ φίλοι σου, οἷς ἐπροφήτευσας αὐτοῖς ψευδῆ" (Jer. 20:6 LXX) - third person pronoun
"ὅτι ψευδῆ αὐτοὶ προφητεύουσιν ὑμῖν πρὸς τὸ μακρῦναι ὑμᾶς ἀπὸ τῆς γῆς ὑμῶν." (Jer. 34:10 LXX; similar 34:14, 34:15, 34:16)
"ὅτι ἄδικα αὐτοὶ προφητεύουσιν ὑμῖν ἐπὶ τῷ ὀνόματί μου, καὶ οὐκ ἀπέστειλα αὐτούς" (Jer. 36:9 LXX)
"Ἀπόστειλον πρὸς τὴν ἀποικίαν λέγων Οὕτως εἶπεν κύριος ἐπὶ Σαμαιαν τὸν Νελαμίτην Ἐπειδὴ ἐπροφήτευσεν ὑμῖν Σαμαιας, καὶ ἐγὼ οὐκ ἀπέστειλα αὐτόν, καὶ πεποιθέναι ἐποίησεν ὑμᾶς ἐπ᾿ ἀδίκοις" (Jer. 36:31 LXX)
"καὶ ποῦ εἰσιν οἱ προφῆται ὑμῶν οἱ προφητεύσαντες ὑμῖν λέγοντες ὅτι Οὐ μὴ ἔλθῃ βασιλεὺς Βαβυλῶνος ἐπὶ τὴν γῆν ταύτην" (Jer. 44:19 LXX)
"ὅτι ὁ ἄνθρωπος οὗτος οὐ χρησμολογεῖ εἰρήνην τῷ λαῷ τούτῳ ἀλλ᾿ ἢ πονηρά" (Jer. 45:4 LXX) - here a different verb than propheteuo is used but still translated 'prophesy'
"Καὶ τίς εἶ σύ, Αχιωρ καὶ οἱ μισθωτοὶ τοῦ Εφραιμ, ὅτι ἐπροφήτευσας ἐν ἡμῖν καθὼς σήμερον καὶ εἶπας τὸ γένος Ισραηλ μὴ πολεμῆσαι," (Judith 6:2 LXX) - spoken pejoratively by Assyrians to Achior, who in context gave an authentic prophecy against them
"λεγοντες προφητευσον ημιν χριστε τις εστιν ο παισας σε" (Matt. 26:68)
Difficult cases/possible counter-examples:
"καὶ ἐπροφήτευσεν Ελιεζερ ὁ τοῦ Δωδια ἀπὸ Μαρισης ἐπὶ Ιωσαφατ λέγων..." (2 Chr. 20:37 LXX)
"Καὶ ἐπροφήτευσεν Αγγαιος ὁ προφήτης καὶ Ζαχαριας ὁ τοῦ Αδδω προφητείαν ἐπὶ τοὺς Ιουδαίους τοὺς ἐν Ιουδα καὶ Ιερουσαλημ ἐν ὀνόματι θεοῦ Ισραηλ ἐπ᾿ αὐτούς" (Ezra 5:1 LXX)
"καὶ εἶπεν Ιερεμιας πρὸς τοὺς ἄρχοντας καὶ παντὶ τῷ λαῷ λέγων Κύριος ἀπέστειλέν με προφητεῦσαι ἐπὶ τὸν οἶκον τοῦτον καὶ ἐπὶ τὴν πόλιν" (Jer. 33:12 LXX)
"καὶ ἄνθρωπος ἦν προφητεύων τῷ ὀνόματι κυρίου, Ουριας υἱὸς Σαμαιου ἐκ Καριαθιαριμ, καὶ ἐπροφήτευσεν περὶ τῆς γῆς ταύτης κατὰ πάντας τοὺς λόγους Ιερεμιου" (Jer. 33:20 LXX)
"Οἱ προφῆται οἱ γεγονότες πρότεροί μου καὶ πρότεροι ὑμῶν ἀπὸ τοῦ αἰῶνος καὶ ἐπροφήτευσαν ἐπὶ γῆς πολλῆς καὶ ἐπὶ βασιλείας μεγάλας εἰς πόλεμον" (Jer. 35:8 LXX - Jeremiah is speaking here referring to true prophets of old)
"καὶ εἰς τὸν συγκλεισμὸν Ιερουσαλημ ἑτοιμάσεις τὸ πρόσωπόν σου καὶ τὸν βραχίονά σου στερεώσεις καὶ προφητεύσεις ἐπ᾿ αὐτήν" (Ezek. 4:7 LXX)
"Υἱὲ ἀνθρώπου, ἰδοὺ οἶκος Ισραηλ ὁ παραπικραίνων λέγοντες λέγουσιν Ἡ ὅρασις, ἣν οὗτος ὁρᾷ, εἰς ἡμέρας πολλάς, καὶ εἰς καιροὺς μακροὺς οὗτος προφητεύει" (Ezek. 12:27 LXX) - here the preposition is eis and thus the noun is in accusative case
"Υἱὲ ἀνθρώπου, προφήτευσον ἐπὶ τοὺς προφήτας τοῦ Ισραηλ..." (Ezek. 13:2 LXX)
"Υἱὲ ἀνθρώπου, προφήτευσον ἐπὶ τοὺς ποιμένας τοῦ Ισραηλ..." (Ezek. 34:2 LXX)
"Καὶ σύ, υἱὲ ἀνθρώπου, προφήτευσον ἐπὶ τὰ ὄρη Ισραηλ..." (Ezek. 36:1 LXX)
"διὰ τοῦτο προφήτευσον ἐπὶ τὴν γῆν τοῦ Ισραηλ..." (Ezek. 36:6 LXX)
"καὶ εἶπεν πρός με Προφήτευσον ἐπὶ τὰ ὀστᾶ ταῦτα..." (Ezek. 37:4 LXX)
"καὶ εἶπεν πρός με Προφήτευσον, υἱὲ ἀνθρώπου, προφήτευσον ἐπὶ τὸ πνεῦμα" (Ezek. 37:9 LXX)
"...καὶ εἶπεν κύριος πρός με Βάδιζε προφήτευσον ἐπὶ τὸν λαόν μου Ισραηλ." (Amos 7:15 LXX)
"Ἐν δὲ τῷ δευτέρῳ ἔτει τῆς τοῦ Δαρείου βασιλείας ἐπροφήτευσεν Αγγαιος καὶ Ζαχαριας ὁ τοῦ Εδδι οἱ προφῆται ἐπὶ τοὺς Ιουδαίους τοὺς ἐν τῇ Ιουδαίᾳ καὶ Ιερουσαλημ ἐπὶ τῷ ὀνόματι κυρίου θεοῦ Ισραηλ ἐπ᾿ αὐτούς" (1 Esdras 6:1)
"υποκριται καλως προεφητευσεν περι υμων ησαιας λεγων" (Matt. 15:7 cp. Mark 7:6)
"και λεγει μοι δει σε παλιν προφητευσαι επι λαοις και εθνεσιν και γλωσσαις και βασιλευσιν πολλοις" (Rev. 10:11)
"καὶ εἶπαν οἱ ἱερεῖς καὶ οἱ ψευδοπροφῆται πρὸς τοὺς ἄρχοντας καὶ παντὶ τῷ λαῷ Κρίσις θανάτου τῷ ἀνθρώπῳ τούτῳ, ὅτι ἐπροφήτευσεν κατὰ τῆς πόλεως ταύτης" (Jer. 33:11 LXX) - prepositional construction, spoken by false prophets of Jeremiah. However it could perhaps be argued that the narrator is using this construction to emphasize that Jeremiah was a true prophet?
"καὶ νῦν διὰ τί συνελοιδορήσατε Ιερεμιαν τὸν ἐξ Αναθωθ τὸν προφητεύσαντα ὑμῖν" (Jer. 36:27 LXX) - dative construction. In the text used by Brenton (quoted here), the question is phrased positively, asked by God to Shemaiah -- thus the implication is that Jeremiah's prophecy is authentic. However, in the NETS, as in the MT, the question is phrased negatively, spoken by the false prophet Shemaiah to the priests in Jerusalem: "And now why have you not reviled Jeremias of Anathoth who prophesied to you?" (I would assume the critical text used by NETS translators differs from that used by Brenton)
"καὶ εἶπεν βασιλεὺς Ισραηλ πρὸς Ιωσαφατ βασιλέα Ιουδα Οὐκ εἶπα πρὸς σέ Οὐ προφητεύει οὗτός μοι καλά, διότι ἀλλ᾿ ἢ κακά" (1 Kings 22:18 LXX) - uses dative construction
"καὶ ἐγὼ ἐμίσησα αὐτόν, ὅτι οὐκ ἔστιν προφητεύων περὶ ἐμοῦ εἰς ἀγαθά" (2 Chr. 18:7; cp. 18:17 similar) - In 1 Kings 22:18 LXX, as you pointed out, the dative is used of apparently authentic prophecy. Even more telling is the fact that the parallel account in 2 Chr. 18:7 uses the prepositional construction. This suggests that, at least for this writer, they were interchangeable.
"προφῆται τοῦ Ισραηλ οἱ προφητεύοντες ἐπὶ Ιερουσαλημ καὶ οἱ ὁρῶντες αὐτῇ εἰρήνην, καὶ εἰρήνη οὐκ ἔστιν, λέγει κύριος" (Ezek. 13:16 LXX) - prepositional construction used although God is referring to false prophets
My tentative conclusion would be that there is a tendency for the prepositional construction to be used more of authentic prophecy, and the dative construction of inauthentic, but that this is driven by contextual factors rather than being a 'rule' or 'idiom' that the writers are consciously using. It cannot be said that the distinction is always important, given the counter-examples above. Furthermore, the Greek of the LXX is older than that of the NT, and we have very few cases in the NT from which to draw.
Finally, I would agree with what others have said, that the context of Jude and 1 Enoch go against the idea that Jude uses the dative to cast aspersions on 1 Enoch. It is worth noting that even Augustine, who had a low view of 1 Enoch and its Watchers story, still affirmed that Jude cited 1 Enoch positively:
"For though there is some truth in these apocryphal writings, yet they contain so many false statements, that they have no canonical authority. We cannot deny that Enoch, the seventh from Adam, left some divine writings, for this is asserted by the Apostle Jude in his canonical epistle." (The City of God 15.23)
Thanks again to all for your input so far.