The ultimate confirmation if any problem diagnosis is, "Does the repair of the diagnosed cause correct the original observed problem?"
On Tuesday, I ordered a replacement for one of the Mercedes' two ignition coils, along with a new ignition harness since one of the spark plug boots was damaged (the extension puller was broken off). Both came from an online import parts store, eGermanParts. They shipped me the parts second-day air for about half the list price of the same parts through a dealer (just under $60 for the coil, about $80 for the complete ignition harness).
The coil and harness arrived this afternoon. It only took about ten minutes to put them in, after which I took the car for a test drive down into Merrimack, then up to Bedford -- not a long run, but with the suspect coil in place, it would have been coughing and stumbling by the time I passed Shaw's Supermarket about a quarter of the way into the trip. It ran smooth as silk all the way to Bedford and back.
I reiterate: "Vacuum leak, my ass."