Q. What if one didn’t have in mind the mitzva while eating on Erev Yom Kippur, does he comply? If he overate it it still a mitzva?

A. According to the opinions that it is a Biblical mitzva (Mishna Berura 604: 1, quoting Magen Avraham) one should indeed have a mitzva intention in order to comply with it. However, Horav Shlomo Miller’s Shlit’a opinion is that after the fact one may still comply, due to the eating being linked to the fasting on Yom Kippur, which is prohibited even without intention.
The Rov maintains that if one overate to the point of being considered an “achila gassa,” whatever was eaten after reaching that point, may indeed be of no value regarding this mitzva.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a