Dear Rabbis: Why is a custom to eat nuts on Seder night? Also, what time in the night is the latest we can eat the afikoman?

The Mishna Berura brings that in order that the children will ask questions one gives them nuts. The Halachic hour of midnight is the latest time for eating the afikoman - Check the calender for the exact hour. In the case you missed the hour, there are Poskim who hold you can eat it also after the named hour. This is only relevant to the first Seder. Rabbi Ro'i Margalit