Yes μακρότερον adverbial, with αἰτιᾶσθαι. (Neut.sing. comparatives double as adj. or adv.)
And yes ἐς ὅσον also adverbial. Lit. “to as much as it’s beneficial.” ξυμφέρει impersonal.
So “Why should we blame past actions at greater length than to the extent that it’s beneficial?” i.e. We should criticize only as much as it’s helpful to do so.
τοις νυν by itself could be dative of τα νυν (“present circumstances” or “current affairs” or the like) or of οἱ νῦν (“people today” or the like). Here evidently the latter (since one benefits people not things).
I don’t why I said τοῖς νῦν is masculine. It’s quite obviously neuter, the present as opposed to τὰ προγεγενημένα, and if I’d only looked at the context the next sentence makes it even clearer. Hugh, you must have seen that I was talking through my hat. You should have said!