Does the future form ἔσται at Luke 21:24 indicate that the trampling would begin at Jerusalem’s destruction? Or does the form imply that the trampling would continue at Jerusalem’s destruction, having already begun at an earlier point prior to that destruction?
ἔσται πατουμένη means fairly the same as the the passive future πατηθήσεται, so the verb form itself does not imply anything about when the trampling begins or continues (except that it happens in the future, of course) or anything at all about the destruction of Jerusalem. I see some commentaries put much weight on the different ways of expressing the future passive here, so that ἔσται πατουμένη should mean “be trampled and continue to be trampled”, but I think that is to read too much into it. And by the way, that meaning is ensured by what follows “ἄχρι οὗ πληρωθῶσιν …” anyway.
Yes, It seems that the phrase itself does not indicate specifically whether the trampling would have already been going on for some time or would just start at Jerusalem’s destruction.
Agreed. The synthetic future (πατηθήσεται) would be aspectually neutral; the future periphrastic is durative. Some good linguistic analysis of verbal periphrasis in Klaas Bentein’s “Verbal Periphrasis in Ancient Greek” and several articles, also Theodore Markopoulos, “The Future in Greek.” (Not so much in NT commentaries, as Πολεμιστής pointed out).
Yes, future form of εἰμί + pres ptcp is typically durative, as here; with an aor ptcp typically perfective (this latter construction becomes more common later).
There is a similar construction only a few sentences earlier in Lk, with the same switch from the neutral synthetic future to the progressive periphrastic: