Laskern,
1. In that case Rav Moshe cannot argue with Rav Yosef Etlinger. However, he can and an Acharon who is of sufficient stature can argue with a Rishon. For that matter, rishonim argue with Gaonim. The Mishna and Gemara are different because there was an acceptance that they are closed (although some Savoraim were added to the Gemara and see also Chochmat Shlomo on Sanhedrin 52b.
2. Why would Tosafot not want to rely on it? Chochma b’goyim taamin. Moreover, Chazal rely on it in hte discussion of a round sukka. See also Tosafot on Eruvin 14a d”h v’haicha where they bring a difficulty from Greek geometry (see Rambam’s commentary on the mishna where he resolves it (there is also a way if one uses an inner an outer border for the yam).