M=Mono or mid channel = L+R/2 = all that is common to both channels
S= side channel or width and position of the mono channel=L-R/2
It is interesting to apply effects–e.g. limiter, compressor–in this “domain” before the inverse transformation.
The so called rotation is rather a conceptional thing. The rotation axis is perpendicular to the horizontal plane between the speakers.
In the channel mixer, there are presets for 90 ° rotations (e.g. “Center to Left”).
Coincidently, it is also the transformation from L/R to M/S
See also Karaoke, Rotation, Panning & more
If I read your article correctly (I understand perfectly German), you’ve already incorporated some delays in the DSP.
Therefore, the center should already be adjustable to stay right in front.
I can not say how stereo widening will affect the sound, especially when the width goes over 100 %.
Keep us updated.