A - you can use cell phone lights but to achieve 1kW/ m2 you should use a lot of them, or they should be very close and illuminate small area of the solar cell.

B - better to use Xe lamps because of better solar spectra simulation

C - The key difference is spectral distribution.

To find how this difference influence to solar cell testing accuracy:

(1) Find white LED spectral distribution (typical spectra could help)

(2) Find Xe lamp spectral distribution (typical spectra could help)

(3) Find solar cell spectral sensitivity (quantum efficacy)

Compare all three spectra.

Using this three spectra calculate weighted flux (irradiation on solar cell) to determine basic design parameters for your sun simulation source.

D - use an opportunity and learn more basics of radiometry and photometry and its application to solar cell energetic efficacy.

Thanks for your response.