A genitive or dative antecedent, when incorporated, usually attracts the relative to its own case.
περὶ δ' οὗ πρότερον . . . ἔθηκε νόμου διελθών (for τοῦ νόμου δν)
a. But a genitive or dative antecedent, when incorporated, is attracted into the case of a nominative relative. Thus, ἐν δικαστηρίοις καὶ ὅσοι ἄλλοι δημόσιοι σύλλογοι (sc. εἰσί)
b. When an antecedent in the genitive or dative is incorporated, the place of the antecedent is usually taken by a demonstrative pronoun in the genitive or dative. Thus,
οὐδέ νυ τῶν περ μέμνηαι, ὅσα δὴ πάθομεν κακά
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].