1) There is a separate component in OptiSystem called LOS Channel, which can be used for the LOS case, while you may use the LiFi Channel components for diffused channel.
2) You may use the swept feature in OptiSystem where you can change the range in step in different ways (Spread Tools) set in the component Sweep Parameters popup window.
You need to make sure that the system still works for any range you designed it to operate over it.
Question: What controls the range in your design? is it a moving object?