1) For a laser the bandwidth (linewidth) is a physical characteristic of the device. Generally a single mode very narrow linewidth is desired but not always possible. This parameter does not depend on the bit rate though. However, modulating a laser source changes the frequency spectrum by generally broadening it which is proportional to the bit rate. This is more important for designing filters and WDM mux’s later though.
2) Yes averaging between the 3 users will give a more accurate value for BER.
Regards