Since I am no tech expert, I would jump to your last question:

When the self-levelling suspension on my 300TD failed, I ask around to see if I could get away with a conversion to a regular step-up like the one the sedan uses. Well, according to every expert, it is impossible to do so. So, I'll guess it would be also impossible to switch a to an air regulated level control.

The rear geometry of the suspension from the T and sedan series is completely different (due to cargo compartment clearance). Also, almost every component is different.

A. Rosich
S320, 1998
E320T, 1995