Material made of combined trans and mirror vs. BRTDfunc

Dear list,
3 questions -
I am modelling solar shading of glass panes. I have modelled them as a
combination of trans and mirror material type. The mirror material is
applied in order to take into acount the specular reflections in the pane in
direct sun. However now I have come in doubt, whether this approach is good
enough. The lamellas will be tiltet in different positions in front of the
facade and interreflections between the lamellas will occur. I want to
include the effect of the interreflections in my model.

1) Therefore question 1 is - does trans and mirror in some way take into
account the variation of reflectance and transmittance for different
incident angles? I have tried to test my "sample" with a glow sky where I
rotate the sample around the x axis. However when I am above a tilt of 45
degrees I get a reflectance of 0... ( My sample have the dimensions 3.5 x
3.5 in x and y and my rtrace point is placed close to the x axis (0.2))

2) To include the spectral dependency with different incidence angles I need
to use the BRTDfunc instead - right?

3) And as the lamellas are made of one glass pane I can use the for defining my material?