Is relative or absolute irradiance mode needed when comparing two scans?
No. Measurements taken with a single spectrometer are accurate relative to one another, even if the spectral shape is uncorrected. That means you can take the ratio of one emissive measurement to another and get an accurate change in the percent signal as a function of wavelength.
Relative irradiance mode is needed when comparing measurements taken by different spectrometers, when determining the spectral shape, or when looking for peak location and shifts. Absolute irradiance mode is needed to quantify the amount of light.