I wonder why they haven't simply renamed the course. If it was called "Battle Magic", would V's curse still apply? What if it was something completely new, like "Sunshine Course"? What if the name was changed to, say, "Transfiguration"? Let the students have McGonagall's Transfiguration and Quirell's "Transfiguration". Students might get a little confused on their first week, but the benefit of having the actual education outweighs the cost... (and btw it's legal to have 2 professors of the same subject -- in canon HP they had Trelawny and Firenze both teaching Divination; not to mention that the headmaster is free to create new positions, like he did with The Keeper of Keys and Grounds at Hogwarts).