Yevamos 20b - 21a - Yibum Works Bidieved

The gemara suggests that yibum is considered "efshar lekayem shnayhem" by doing chalitzah, and then asks "im ba'alu kanu" - meaning that if there were no mitzvah of yibum when chalitzah can be done, yibum should not even work to be koneh bidieved. It seems that the gemara has 2 assumptions: 1. the kinyan of yibum is dependent on the fulfilling of the mitzvah. 2. the concept of efshar lekayem shnayhem does not only create an issur to to yibum but rather makes it that you have not fulfilled the mitzvah of yibum at all. Therefore the gemara asks, that since yibum is koneh bidieved, it must be that chalitzah is not called efshar lekayem shnayhem.

This seems plausible by the concept of efshar lekayem shneyhem, but this will not explain the gemaras question 21a by an aseh not pushing off a lo sa'aseh and aseh. The shita of the rivah is that the aseh does in fact push off the lo sa'aseh (i.e. no malkus), but it is assur to do it since the aseh of yibum does not push off the aseh. It seems clear that acc. to the rivah, if one were to do the mitzvah of yibum, although it is assur to do so, they did in fact fulfill the mitzvah, so why does the gemara say that if one would do yibum to an almanah min hanesuin, it does not exempt the tzarah, which implies that the mishnah was not fulfilled?

sorry for being brief - i am in jerusalem now and don't have that much computer time.