R’ Shamshon Raphael Hirsch seems to indicate that the Mekoshesh happened right after the yidden were taught Hilchos Shabbath in Marah which is before Matan Torah. Moshe knew that he was to receive the death penalty just not which type.
He also explains that the question of Bnos Tzelaphchad was not about the general halachos of inheritance but of the special situation about going into Eretz Yisroel and the division of land based on who left Mitzrayim and what if that person had no sons. That specific question may not have been addressed.
Pesach Sheini is a good question since it was definitely after Matan Torah.