I do not understand what kind of ἀλλά this is: ἀλλὰ ταῦτα περὶ σοῦ καὶ οἴκαδε ἀπαγγέλλωμεν; I am not sure whether Smyth refers to this type of αλλα in 2784e.
NB is this the same α’λλά as in Eur.Med 326: ἀλλ᾽ ἐξελᾷς με κοὐδὲν αἰδέσῃ λιτάς (will you banish me being not ashamed of my supplications?). In my old Russian grammar of Greek this α’λλά is rendered as разве.
It’s a normal enough use of ἀλλά, in that it contrasts one thing with another. If it helps, you can think of it as elliptical: “but instead (of learning what you say arete is), are we to report …?”
The use in the Medea passage is somewhat comparable: “but instead (of my persuading you), will you drive me out …?” It’s indignant and pathetic.
You have to grasp the basic idea of ἀλλά, regardless of how it’s translated.
And you should have given the immediately preceding text, since ἀλλά always marks a contrast with what comes before it.