Laskern,
1. The Behag paskens that the Torah requires us say Hallel on a day when the deliverance occurred but the other observances are rabbinic. If Jews in Chul do not observe an extra day of Rosh Chodesh, which is completely d’Oraita, they certainly will not in the future (if there are Jews in Chul after Mashiach comes). Actually even the second day of Yom Tov is actually a minhag being that we have a fixed calendar (Beitza 4b).
2. a. Who says it only applies if it comes later? b. Actually it was never a safek as the shlichim certainly reached Bavel by the 25th.