Pelican Technical Article: Transmission Mount Replacing
https://www.pelicanparts.com/techarticles/Mercedes-W211/109-TRANS-Transmission_Mount_Replacing/109-TRANS-Transmission_Mount_Replacing.htm
Looks like the captured nut for the red bolt is inside a small formed steel hanger (top center of below photo), and the bolt needs to come out so you can remove the crossmember from the body and trans mount bracket. If the nut spins, maybe you can jam something between the nut and the side of the hanger to hold it in place well enough to loosen the bolt.
