Update,
Yet again a
new piece of return line came off the injector hose barb. I did several miles of test driving with fuel pressure gauge connected inline with return line. I never saw the pressure go above 7. (Some of this test driving was during the SBC brakes red-dash-of-death issue...see my other thread on that).
I found another thread about replacing return lines with PFA Teflon tubing. I procured all the tubing and clamps per that thread, and have had no fuel leaks since. That tubing is rated for a wide range of temps, and a wide range of chemicals. About 1000 miles on new return lines now.
The thread was from a different forum. Here's the link if the moderators will allow it:
http://www.benzworld.org/forums/w211-e-class/2036961-e320-cdi-return-fuel-line-replacement.html