correct me if Im wrong but „Diffuse” in corona mat describes total reflectance (diff + direct). when you add „Reflection” on top of that Corona dimms diffuse part to balance the energy.
No, the Diffuse in CoronaMTL is only the diffuse albedo part, not total albedo (Diffuse reflection albedo+ Specular reflection albedo). Second part is correct about Energy Preservation.
But energy preservation in shader doesn't account for scanned F0 values. There is some arbitrary deduction from early version ( two years ago ?) done on diffuse but this is done on top and it's just hack.
It does basically the same thing, reducing the diffuse albedo slightly even before specular is applied. The amount is smaller than 4perc. though.
Same goes for using photometric stereo scanned textures. Energy preservation will only kick afterwards but for the shader to recreate exact albedo you need to feed the diffuse slot only pure 100perc. diffuse albedo.
( additional cross-polarization scan to extract specular scan and then the classic 50perc. negation to get pure diffuse albedo).