. . τῶν δὲ Ἀθηναίων ἔτυχε γὰρ πρεσβεία πρότερον ἐν τῇ Λακεδαίμονι περὶ ἄλλων παροῦσα, καὶ ὡς ᾔσθοντο τῶν λόγων, ἔδοξεν αὐτοῖς παριτητέα ἐς τοὺς Λακεδαιμονίους εἶναι, τῶν μὲν ἐγκλημάτων πέρι μηδὲν ἀπολογησομένους ὧν αἱ πόλεις ἐνεκάλουν, δηλῶσαι δὲ περὶ τοῦ παντὸς ὡς οὐ ταχέως αὐτοῖς βουλευτέον εἴη, ἀλλ᾽ ἐν πλέονι σκεπτέον
ἔδοξεν αὐτοῖς – this is the verb that introduces indirect speech: “it seemed best to them,” “they decided” that
παριτητέα ἐς τοὺς Λακεδαιμονίους εἶναι, – the main clause of the indirect speech; the tense is present, in relation to the time of the main clause. παριτητέα ἐς τοὺς Λακεδαιμονίους here means something like “make an appearance” or “step up to the speaker’s platform” before the Lacedaemonian assembly. παριτητέα is a verbal adjective expressing necessity. It’s neuter plural here; neuter singular is more usual.
τῶν μὲν ἐγκλημάτων πέρι μηδὲν ἀπολογησομένους ὧν αἱ πόλεις ἐνεκάλουν – a future participial phrase within the indirect speech. The future participle, as usual, indicates purpose. Presumably ἀπολογησομένους agrees with the understood agent of παριτητέα, the Athenian envoys, but normally the agent of a verbal adjective is dative. Perhaps the accusative here referring to the Athenians helps avoid confusion later in the sentence where dative αὐτοῖς with verbal adjectives refers to the Lacedaemonians.
τῶν μὲν ἐγκληματῶν πέρι . . . ὧν αἱ πόλεις ἐνεκάλουν – the verb of the subordinate relative clause is indicative because the clause is to be read as an explanatory statement of fact independent of the thought processes of the Athenians. ὧν is “attracted” into the case of its antecedent, τῶν ἐγκλημάτων. It’s the internal object of ἐνεκάλουν (ἐγκλήματα ἐνεκάλουν). In independent clauses, the internal object of a verb is accusative.
δηλῶσαι δὲ περὶ τοῦ παντὸς ὡς is part of the indirect speech introduced by ἔδοξεν αὐτοῖς, and it introduces another level of subordination/indirect discourse. With infinitive δηλῶσαι δὲ, Thucydides has switched from the future participle ἀπολογησαμένους μὲν depending on the understood accusative agent of παριτητέα to infinitive δηλῶσαι subordinated directly to ἔδοξεν αὐτοῖς, avoiding strict parallelism despite the μὲν . . . δὲ contrast, as Thucydides likes to do.
ὡς οὐ ταχέως αὐτοῖς βουλευτέον εἴη, ἀλλ᾽ ἐν πλέονι σκεπτέον – again, this is second-level subordinated indirect discourse. Two verbal adjectives in -τέος, expressing necessity, this time neuter singular instead of plural. (Perhaps singular instead of plural is another way to avoid confusion with the previous verbal adjective, παριτητέα.)
Here the verb is optative, εἴη, because the clause is subordinated within the indirect speech introduced by ἔδοξεν αὐτοῖς and is to be read as part of and not independent of the thought processes of the Athenians. If instead of δηλῶσαι in indirect speech, this clause were introduced by a verb in direct speech in a main clause, let’s say δηλοῦσιν, the verb of the ὡς clause would be indicative: δηλοῦσιν ὡς οὐ ταχέως αὐτοῖς βουλευτέον ἐστὶν, ἀλλ᾽ ἐν πλέονι σκεπτέον. The optative makes it clear that the ὡς clause is part of the Athenians’ thinking, not explanatory material added by Thucydides.
However, the optative is, well, optional: it would be equally permissible to cast this clause in the indicative, retaining the mood of a main clause in direct speech, but changing the tense of ἐστίν to imperfect ἦν to bring it into the time frame of the narrative: ἔδοξεν αὐτοῖς . . . δηλῶσαι ὡς οὐ ταχέως αὐτοῖς βουλευτέον ἦν, ἀλλ᾽ ἐν πλέονι σκεπτέον.
That’s all I have. I hope it’s not too difficult to follow.