If the usbasp can flash the firmware, it can also set the fuses. But if the MCU is very underclocked or uses the reset pin, it could require high-voltage reprogramming.

As for learning, there is a fairly significant amount of example code in the repository to learn from… and for making e-switch UIs, there’s also a UI toolkit which eliminates most of the need to touch low-level code. The toolkit takes a significant chunk of ROM, but it makes the rest of the process much easier and may also reduce code size overall compared to older coding styles.