ἀλλ᾽ ὡς περὶ κλωπῶν βουλεύεσθαι
I do not see how it may be connected to the rest of the sentence: εἴ τινι ἐντυγχάνοιεν τῶν Ἀρμενίων, τοὺς μὲν ἂν συλλαμβάνοντες αὐτῶν κωλύοιεν τῶν ἐξαγγελιῶν, οὓς δὲ μὴ δύναιντο λαμβάνειν, ἀποσοβοῦντες ἂν ἐμποδὼν γίγνοιντο τοῦ μὴ ὁρᾶν αὐτοὺς τὸ ὅλον στράτευμά σου, ἀλλ᾽ ὡς περὶ κλωπῶν βουλεύεσθαι.
Also, ἄν is omitted with δύναιντο, but it is the same opt potentialis as the rest.
I agree – ἀλλ’ ὡς περὶ κλοπῶν βουλεύεσθαι isn’t connected with the rest of the sentence syntactically.
οὓς δὲ μὴ δύναιντο λαμβάνειν, ἀποσοβοῦντες ἂν ὲμοδὼν γίγνοιντο τοῦ μὴ ὁρᾶν . . . This is a future general relative clause – like a future less vivid condition – optative without ἄν in the protasis, optative with ἄν in the apodosis.
The old commentary says: we must evolve some positive verb, such as ποιοῖεν, from the negative ἐμποδὼν γίγνοιντο. Then it adduces another example from Hell., V I 21. So if we reconstruct, the sentence should be like: οὓς δὲ μὴ δύναιντο λαμβάνειν, ἀποσοβοῦντες ἂν ἐμποδὼν γίγνοιντο τοῦ μὴ ὁρᾶν αὐτοὺς τὸ ὅλον στράτευμά σου, ἀλλὰ ποιοῖεν τοῦ ὡς περὶ κλωπῶν βουλεύεσθαι.
However, as an afterthought, I think it is not really needed and βουλεύεσθαι is linked to ὁρᾶν.
No, your commentary is right. There is some notional finite verb in the optative underlying βουλεύεσθαι. “. . . but instead [they would be induced] to plan/take measures against thieves.” The missing verb is parallel to ἂν ἐμποδὼν γίγνοιντο, and needs another ἂν. The idea is that the Armenians who escaped would be prevented from seeing the size of the army, but after seeing the scouts disguised as brigands, would be led to take measures to deal with thieves, rather than preparing for an invasion.
sure in this case ἄν is needed it just slipped my mind.