too keep it simple
Unmixing methods is based on assumption that pixel spectral value in an image generally coming from multiple objects in the earth surface. unmixing methods tried to separate/divide spectral information in a pixel into portions of spectral donation from every object, using field based/in situ spectral response measurements.
whereas, radiative transfer models tried to measure how much the rest of electromagnetic energy (as the result of interaction between electromagnetic iradiance versus surface objects reflectance), when the radiance is arrived at the sensor (top of atmosphere).
So unmixing is about proportion and radiative transfer is about how much, In this matter radiative transfer is more robust and physical based than unmixing, but doesnt always superior in the application level.
yes they could be used in biophysical extraction, both of them.
yes but they could be applied to satellite based imaging too,