from a reader michael post
When talking about the month of Nisan and the extra Adar, when it is considered whether or not messengers would need to be sent out in the second Adar, Tosefos comments that one would expect that messengers would need to be sent ANYWAY just to inform them that the month was in fact Adar Sheni instead of Nisan so that they don't celebrate Pesach at the wrong time. The answer given there is that they would expect the Nisan messengers to arrive telling them the appropriate day. The ABSENCE of the messengers therefore informs them that the month is in fact Adar Sheni.
Based on this, I see two possibilities:
a) Everyone lives within the area that messengers can travel within 15 days. Therefore, the absence of messengers will in fact indicate to people that the month is Adar Sheni. However, since everyone is notified in time, there should never need to be 2 days of Pesach at the beginning (and at the end) out of uncertainly, since everyone knows what day it starts.
b) Not everyone lives within the area that messengers can travel within 15 days. That explains why there are two days for Pesach outside of Eretz Yisrael, but it does NOT explain why messengers are not sent out saying that the second month was Adar Sheni. People outside of the 15 day range will in fact be observing Pesach at the wrong time and, to make matters worse, by not sending out messengers informing of Adar Sheni, and since the Nisan messengers will not arrive until after Pesach, they will not even have the chance to find out that they should in fact observe Pesach again the following month!
avrohom adler, usa
TUREI EVEN ( DH M"S), cited by RABBI AKIVA EIGER, asks that if after Purim the Beis Din decided to add Adar Sheni, the messengers should go out to inform those living 14 days away from the place of the Beis Din that it had been made into a Leap Year, because people there would think that really it was Nisan, but that the messengers had simply not had enough time to reach them by the 14th of Nisan (and therefore in these distant places they would mistakenly keep Pesach when it was really Purim for the rest of the world).
RITVA appears to have anticipated the question of Turei Even, because he cites Tosfos' question and answer, and adds that "it would be well-known that an extra month had been added, even in places where the messengers did not reach". Ritva does not explain how these places would know about it without the help of the messengers. Possibly the locals were able to travel faster, because of their superior knowledge of the local terrain, than the messengers of Beis Din (or maybe they possessed some kind of primitive telephone system!)
(All of the above problems only apply according to Tosfos' position that messengers were not sent out to publicize that a Leap Year had been announced. However Ritva writes that if the Leap Year was announced after Purim, the messengers left immediately to publicize this, so everyone knew about this before they might think that it was Pesach. These messengers were not mentioned in our Mishnah because the subject of Maseches Rosh Hashanah is to teach how the messengers announced whether Rosh Chodesh was on the 30th or 31st day, not to deal with the messengers who publicized the Leap Year. This is stated also by PNEI YEHOSHUA and RASHASH).
Here is another approach to answer your question. It may be simply that Chazal did not go to such lengths to cater for those so far away from Yerushalayim i.e. 15 or more days travelling time. (In addition it is quite probable that the vast majority of Klal Yisrael lived within the 15-day radius so it was not justifiable to institute new measures for the sake of such a small minority, and we could say an even bigger Chidush, that Chazal did not want to make measures for these distant people in order not to encourage them to live so far away).
Therefore it is true that people living further away would keep Pesach and later find out that it really should have been Purim, but since they will be able to keep Pesach again next month at the correct time, the damage done was not so great. At least they did not eat Chametz on Pesach.