Sequential Game Rollback Equilibria Question
You're correct on the definition of the strategies: Albus' strategy set is $S_{A} = \big\{N,E,S\big\}$, and Minerva's is $S_{M} = \big\{a, b\big\} \times \big\{a, b\big\} \times \big\{a, b\big\}$. 

Remember, a strategy is a contingent plan that prescribes an action at every possible node at which a player plays.

Then, the unique equilibrium (via Backward Induction) is $(N, b, a, b)$. The others you mention are not equilibria, because they involve incredible threats, e.g. if Minerva found herself at the decision node following a choice of $E$ by Albus, she would never choose $b$.