Lo Sasur-When Can One Argue

My good friend Chaim Brown has posted about Lo Sasur on his blog over here . In looking up the Rav Elchanan (Kuntres Divrei Sofrim 1:35-39) that he quoted, I noticed another Rav Elchanan that was also interesting. R' Elchanan in Siman 2 discusses how is it possible to argue on a psak if there is an issur of "lo sasur". R' Elchanan explains based on the Rambam in his intorduction to the Yad and the Rambam in the beginning of Hilchos Mamrim that there are two qualifications for a psak to be included in "lo sasur". The first is mefurash in the Rambam in Hichos Mamrim that it has to be a psak coming from the Sanhedrin located in the Beis HaMikdash. (Exactly what type of psak is a different question all together, and maybe I will discuss it later). The second qualification is that if teh majority of the chachamim decide a halacha then one is not allowed to go against it. The source for this is based on the loshon of the Rambam in his hakdamah to the Yad. R' Elchanan then adds that with this we can explain why the amoraim were not allowed to argue on the tanaim. (This is the Kesef Mishna's question in Hil Mamrim Perek 3). Since the chasimas hamishna was established by rov chachamim, it comes out that to argue on the mishna would be a violation of lo sasur. The same reasoning would explain why we can no longer argue on teh gemara after the chasimas hatalmud.