οὐκοῦν εἰ μὲν ἀεὶ εἶχεν, ἀεὶ καὶ ἦν ἐπιστήμων: εἰ δὲ ἔλαβέν ποτε, οὐκ ἂν ἔν γε τῷ νῦν βίῳ εἰληφὼς εἴη. This is a rare past unreal condition, is it homeric? Smyth, 2311-12 If does not look like potential.
How I hate these labels!
οὐκοῦν εἰ μὲν ἀεὶ εἶχεν, ἀεὶ καὶ ἦν ἐπιστήμων presents no difficulty (“if he had it all along, then he was also ἐπιστήμων all along”), so I guess you’re referring to εἰ δὲ ἔλαβέν ποτε, οὐκ ἂν ἔν γε τῷ νῦν βίῳ εἰληφὼς εἴη, which means “while if at some point he had gotten it, he wouldn’t/couldn’t have gotten it within his current lifetime.”
Perhaps you were expecting ἔλαβεν rather than εἰληφὼς εἴη, but the perfect participle is more precise, referring to his being in a state of having gotten it, his being in possession of it by acquisition. Separating εἰληφὼς from εἴη may make it easier for you to grasp, for οὐκ ἂν … εἴη is a regular potential optative: “he wouldn’t be in a state of having gotten it.”
So Plato has (as usual) expressed exactly what he means. I doubt the construction is Homeric.