With all the O2 sensor and egr codes, you may have a plugged hot inlet pipe to the intake manifold. The egr valve is on the exhaust manifold, and when the egr valve opens, it allows egr vapors to travel back to the intake manifold via this hot inlet pipe.
Steve Brotherton wrote an article in the DIY link at the top of the screen called Evaluating Electronic Engine Controls that shows this problem on a C280, which I think is the same engine as yours. The first half or so of this article is too technical for this shadetree mechanic, but this problem is explained at the end, with pictures. This condition seems to be systematic with all M104 engines (including mine).
This pipe gets coked up with carbon from these vapors, and triggers a code for low egr flow.
The fix is to ream out the pipe with a cable, like a speedometer cable, and a drill motor.
I've think I've read a post from Benzmac saying that he performs this every 30,000 miles on M104 engines as part of regular maintenance.
My mileage didn't really suffer through this process, only to the extent that it dropped while the mechanics kept the engine running so long diagnosing it.
My average mpg is about 21 - 21.5 consistently.
I have experienced in other cars significant drops in mpg when my O2 sensors were bad, though.
Don't give up on your problems, though. I had one indie mechanic give up and say he couldn't diagnose my CE light problems. Another indie mechanic blamed it on the car , you know "..sometimes these engines have CE light problems that have no real source...yada yada yada yada". He was wrong, and this website helped me diagnose the problem.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".