The Dutch guy whose comments I read says it is unnecessary to suppose an ellipse of melos with ade ( which LS proposes, c. gen. (sc. μέλος), sing an air of .). He says it’s kinda Gen partitivus or something like that. But I think in such a case it may be Gen of origin as well.
PS: after ὄτι a kind of subjunctive is used (καὶ ἀπορεῖς ὅτι λέγῃς, which looks like deliberative), a little farther: καὶ εὐπορεῖς ὅτι λέγῃς, 536c, is this one deliberative too? after an affirmation? Strangely enough, the commentators are silent on these subjunctives.
Replying to the PS, I thought that ὅτι was ὅ τι / ὅ, τι there.
And Googling for “ὅ τι λέγῃς” returns this discussion of the subjunctive in Goodwin as one of the top hits:
thank you, so this is as I thought, deliberative subjunctive.