Good questions. (1) Rav Zera's Hava Amina was that one must recite a blessing right as one begins to partake in the thing for which he is reciting the blessing. With regard to food, one does not make the blessing when the food is resting on the table, but only when one has begun to eat it -- i.e. when he lifts it up in his hand. Since this type of "beginning" does not exist with regard to smell, Rav Zera has a Hava Amina that one should actually begin to smell it before reciting the blessing.

(2) When Rava answered that ha'Motzi is also recited before one eats the bread, he was not referring only to the blessing over bread. He meant that all blessings for pleasure (Birchos ha'Nehenin) are recited before one experiences that pleasure (we find the same idea, that one blessing is used to refer to all of them, in Rosh ha'Shanah 29a (at the end of the Amud), as is explained in Rashi there. -Rav Eliyahu Guttmacher). (It seems that the reason for having to make a blessing before eating is the Sevara (35a) that one is forbidden to derive pleasure from anything without first reciting a blessing. The concept of Oveir la'Asiyasan, on the other hand, is really only relevant for a blessing on a Mitzvah, which is a Ma'aseh (la'*Asiyasan*).)