Ok, since nobody else answers I'll give it a try. I recently changed the transmission mount, the rear flex disc and the support bearing of the driveshaft on my W124. Based on this experience I'll say that you do not need to remove the transmission support bar but you need to collapse the driveshaft by loosening the big sleeve nut (and unboltung the driveshaft boot). In my case the flex disc was almost baked to the driveshaft and was hard to separate. Sensible use of an air chisel came very handy at this place.
Good luck, Bruno

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1992 300TE 160 kmiles