Q. Would the mitzvah of being the very first person in shul override going elsewhere to answer brachos or kaddishes? Is there a difference between Yom Kippur and other days.

A. Poskim write that it seems from most of the Rishonim that the appreciation and reward for the first ten individuals that create the minyan, is the same for all.
However, Ma’adanei Yom Tov maintains that each one of the first ten, receives recognition for all the remaining of the ten that join after him.

Meir Oz (p. 166) quotes Siach Hassode that the very first one, is indeed above all others. Shulchan Shlomo (1,1) mentions that he is granted the qualities of a Tzadik.
It would be reasonable that the merit is greater during Yom Kippur.

Rabbi A. Bartfeld as revised by, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a