So... Several hundred dollars later.
The (new) O2 sensor checks out, no vacuum leaks, per the 'smoke test'.
Everything checks out OK so far, but it had codes for EGR and O2 sensor.
I pulled the EGR tonight, and the diaphragm is leaking, but the valve itself is tight and not leaking, i.e. not contributing to an internal vacuum leak.
The Benz shop has exhausted the usual suspects, and is looking for a bag 'o dollars to start the next (as yet undefined) stage in diagnostics.
Has anyone had trouble with the throttle position sensor, or whatever its called, under the throttle linkage, and does anyone have a diagnostic routine for it?
Best regards,
Mark