Trial translation: Moreove, gentlemen of the court, the ones who bore malice against your majority, bore it no less in keeping silence. For there were other men, the ones advocating and doing things that were extremely harmful harmful to the city.
This is a free translation that reflects my uncertainty about the Greek grammar of the quotation. Is “οἱ λέγοντες καὶ πράττοντες” an appositive in the Greek? Or should “ἕτεροι γὰρ ἦσαν οἱ λέγοντες καὶ πράττοντες” be rendered “the ones speaking and acting were different”? Is “οὐχ οἷόν τ᾽ ἦν” an impersonal idiom meaning “it was not possible”?
To take your last question first. ὧνοὐχοἷόντ᾽ἦνμείζωκακὰγενέσθαιτῇπόλει “than which it was not possible (yes, οὐχοἷόντ᾽ἦν impersonal) for there to be greater evils for the city.” ὧν (neuter) genitive of comparison with μείζω. If this were an independent sentence it would be τουτων not ὧν, “evils greater than these.”
There’s no expressed antecedent, it’s supplied from the understood object of λέγοντες καὶ πράττοντες: “[things] greater than which …” Cf. your earlier query on θανάτου ἄξια εἰργάσθαι δόξῃ.
οὐδὲνἔλαττονεἶχον I think you understand this, sc. κακονοως, they were no less malignantly disposed.
As for ἕτεροι γὰρ ἦσαν οἱ λέγοντες καὶ πράττοντες, yes, it’s “the ones speaking and acting were different” (ἕτεροι predicative) i.e. others than themselves. This doesn’t let them off the hook, since they were members of the Thirty even if they were not actively involved. The argument is not spelled out in full.