The inferno spell being purchasable from the get-go was intentional, but noted on the game-breaking side effect. Narcissa in that fight was supposed to have plot armor, even against magic, but I missed the "overheat" status. Will patch out soon. Thank you so much!
Also, the "W" button is the same as the "skip button" (aka RB on a controller) used to skip dialogue, and doubles as a skip turn for the battles. Will make it clearer in the keyboard controls tutorial.