One down, one to go...
Well, no one has taken up the challenge of responding to my two questions yet; however, I've found one of the answers on my own by actually measuring the device.
For the record--and for anyone who stumbles onto this thread in the future--the right socket for removing and/or replacing the fuel sending unit is 46mm or 1 13/16".
Still hoping someone can provide me with proper torque specification though...
1991 560SEL-Euro (145,000 miles)
1989 420SEL (Retired at 325,000 miles)