They are coopting the nerves in order to eventually replace them. Think fungal hyphae threading into existing nerves and then, once enough threads are woven together, the nerve becomes almost entirely fungal mycelium. So, the answer is both! At first, Alec would be playing a fun little game of tug-o-war, and then once someone's too far gone, it'll be very difficult to control them using their nerves because their nerves are fungus now. The difficulty would basically scale with the duration of the infection, think getting harder and harder every couple of days.