If the wavelength of the white light laser is chosen to be at the center of the range of wavelengths used for the fiber bragg gratings, there would somehow be a symmetry in the way the wavelengths far from the center receive power. So it had better be chosen that way.
The chosen extinction ratio has been chosen as the default value. But you can change it according to your paper (if there is any mention of it).
Don’t you see any difference in the performance by changing the EDFA gain?