E.C. Marchant, Commentary on Thucydides: Book 3

μετὰ καινότητος—equivalent to a dat, of cause, as I. 32 μὴ μετὰ κακίας, δόξης δὲ μᾶλλον ἁμαρτίᾳ: cf. c. 42, 1.

ἀπατᾶσθαι ἄριστοι— sc. ὄντες, co-ordinate with κακῶς ἀγωνοθετοῦντες, the second way in which you are αἴτιοι: εὐπαρἀγωγος εἶ, θωπευόμενός τε χαίρεις κἀξαπατώμενος, Aristoph. Eq. 1115.

μετὰ δεδοκιμασμένου—sc. λόγου, ‘when an approved argument is stated’; ξυνέπεσθαι means ‘go with the speaker.’ (Another way is to take ξυνέπεσθαι μετά closely together, ‘to follow the lead of’: I prefer the former.)

δοῦλοι ὄντες—the whole down to ἀποβησόμενα is epexegetic of ἄριστοι (ὄντες).