Regarding the second teretz dealing with the two days of ROSH HASHANA, we were wondering if there are other places where a Mishnah is exclussively speaking about Eretz Yisroel as oposed to a more general intent, as you state: "because the Mishnah was written in Eretz Yisrael, where there no other Yom Tov was two days long".
It just seems a little strange - genrally the Mishnayos will be more specific as to intent and applicability.
We find a number of Mishnayos which speak from a perspective of being in Eretz Yisrael -- for example, the Mishnayos at the beginning of Megilah and Gitin. We do find, however, in the eighth Perek of Gittin (79b), a Mishnah speaking from a perspective of Bavel. Commenting on the Mishnah there, Rashi says that the Mishnah is discussing someone who was in Bavel (even though it does not say so explicitly in the Mishnah).
The Mishnayos, actually, are particularly non-specific with regard to intent and applicability (as we see from learning the Gemara on the Mishnayos).
All the best,