How to find how much there is of a compound given the following information?
Maybe I'm just too tired right now, but didn't you mention that the absolute amount of magnesium oxide doesn't change during aqueous workup? 

Isn't thus the amount of 3.22 g $\ce{MgO}$ exactly what was formed initially?

Wouldn't it be more reasonable to let the students calculate how much magnesium metal was burned initially, taking the $\ce{N2}$/$\ce{O2}$ ratio in air and the stoichiometry of the magnesium nitride into account? ;)