Rashi on 45b D"H V'Lakach M'lo says where is from before Hash-m - the west side. - But it's not the entire west side it is only the north west side.
Rashi on 46a D"H Aval M'hai Geisa - says the question is on the part of the west side that is not opposite the door.
My assumption is then that Rashi only had the version of the gemora asking aval m'hai geisa - so the conclusion would be that no the entire west side is okay - (like the Rashi on 45b) but NOT the east side.
Whereas, our version asks aval m'hai geisa umeihai geisa - asking maybe the southwest is not good and maybe the east is not good - so the gemora would be answering - no the entire mizbeach is good.
Mordechai Cutler, Chicago, USA
Assumptions based on Girsa'os are difficult to make.
Your question is based on your mistaken assumption that only the northwest side is Lifnei Hash-m. If Lifnei Hash-m means oppossite the door, then it is definately not the northwest side. The two major Shitos of where the Mizbe'ach was are:
1. Kol ha'Mizbe'ach b'Tzafon. This means that the south end of the Mizbe'ach is k'Neged ha'Pesach (for three Amos, after we subtract an Amah for the Sovev and an Amah for the Yesod). The Gemara here is not following that view.
2. The Mizbe'ach is in the middle. There are variations of this Shitah. However, if the Mizbe'ach is in the middle, then ten Amos (five north and five south) are k'Neged ha'Pesach and 22 are on either side of the door. Of these, 11 are on one side of the door including the Sovev and the Yesod, and 11 on the other side. The Gemara says that if the verse would just say "mi'Lifnei Hash-m" and not say "me'Al ha'Mizbe'ach," we would have thought that the 9 Amos on either side of the Pesach are invalid.
D. Zupnik