if you really think about a rework of the light source input please use lumen [lm] for all light sources except the ambient ones (sky,sun,hdri).
The lumen unit is directly handling the brightnes over color issue due to integrated human eye spectral sensitivity (v(lambda))
It's also photometrical and physics based and can easily be set according to real world lamps and luminaires. Most of them have a note of lumen output on their packaging.
1. You can't define a lumanince variation if you simply set an cd/m^2 value for area lights. But a lumen + IES file could do.Also let's not forget than Physical units are also user-friendly
If you want you area light to maintain its brightness independently of its size, use candela/m2 (a.k.a nits). If you don't, then use candela.
If you want your spotlight to maintain its brightness independently of its size, use candela. If you don't then use lumen.
Lumens are great for pointlights as these values are well-knowns on lightbulb packages, and are based on human perception.
If you want to use radiometric and ignore human eye color sensitivity, then use Power and Efficacy.
2. A single candela value will create an even light distribution for a spot light. Variation is possible with having lumens + IES or maybe lumens + spread angle + falloff.
3. Pointlight in lumen as already mentioned
For the ambient sources you could maybe think about this.
Sun and skies should get something like a scale value. but keeping this at 1 (100%) should deliver natural high luminance values.
For the single color and hdri ambient sources maybe a fixed luminance (cd/m^2) value could be applied. This value should be applied to the brightest (whitest) pixel on the image. That should allow us to work with realistic lighting conditions.
Hope some of this will find it`s way into luxcore.
BR.