My thinking arises from that flowers are probably, and nightingales are certainly standards against which to measure visual and aural beauty.
Following Denniston, and saying, "Daphnis is comely. Yes, and the flowers (are comely too)", sort of turns that assumption that the flowers are the reference point on its head. The comment in the Particles
that "If καὶ γὰρ is ever so used in continuous speech may be questioned", perhaps means that has Chloe in two minds and conversing with herself, or that this is not the καὶ γὰρ of dialogue mentioned by Denniston, but rather that the γὰρ is the beginning of an explanatory phrase and the καὶ is adverbial, "for even the flowers are comely (because Daphnis has given my heart a new sense of appreciation for comeliness).
The example quoted from Against Phormio
, the καὶ γὰρ is a rhetorical response to the somebody quoting a term from a contract, so even though it is not in the form of a dialogue, it is still a reply to something somebody else is portrayed as saying:
Demosthenes 34.33 wrote:λέγει δὲ ὡς ἡ συγγραφὴ σωθείσης τῆς νεὼς αὐτὸν ἀποδοῦναι κελεύει τὰ χρήματα. καὶ γὰρ ἐνθέσθαι τἀγοράσματα εἰς τὴν ναῦν κελεύει σε, εἰ δὲ μή, πεντακισχιλίας δραχμὰς ἀποτίνειν. σὺ δὲ τοῦτο μὲν τῆς συγγραφῆς οὐ λαμβάνεις, παραβεβηκὼς δ᾽ εὐθὺς ἐξ ἀρχῆς καὶ τὰ χρήματα οὐκ ἐνθέμενος ἀμφισβητεῖς πρὸς ἓν ῥῆμα τῶν ἐν τῇ συγγραφῇ, καὶ τοῦτ᾽ ἀνῃρηκὼς αὐτός. ὁπότε γὰρ ἐν τῷ Βοσπόρῳ φὴς μὴ τὰ χρήματα ἐνθέσθαι εἰς τὴν ναῦν, ἀλλὰ τὸ χρυσίον τῷ ναυκλήρῳ ἀποδοῦναι, τί ἔτι περὶ τῆς νεὼς διαλέγει; οὐ γὰρ μετέσχηκας τοῦ κινδύνου διὰ τὸ μηδὲν ἐνθέσθαι.
He says that the agreement bids him pay back the money, “when the ship reaches port in safety.” Yes, and it bids you also to put on board the ship the goods purchased, or else to pay a fine of five thousand drachmae. You ignore this clause in the agreement, but after having from the first violated its provisions by failing to put the goods on board, you raise a dispute about a single phrase in it, though you have by your own act rendered it null and void. For when you state that you did not put the goods on board in Bosporus, but paid the cash to the shipowner, why do you still go on talking about the ship? For you have had no share in the risk, since you put nothing on board.