Of course you can merge M/S channels to L/R files. You have plugins for this in WaveLab (look at the StereoTools). But in the “destructive way”, that is, to rebuild an audio file. The split you mention in the audio montage, is non destructive. Once you apply effects to the M/S channels, you can’t obviously merge them again “non-destructively”.
And Audacity won’t help you ther!