The culprit was the first forward hydraulic lifter/compensator on the passenger side. Interestingly, this is the first failed lifter my mechanic has seen in his 40+ years of working on Mercedes.
This was also the first time I have seen the inside of my engine, and it is safe to say that given what I observed, replacing lifters & rocker arms, etc. is a quick DIY job. The only apparent gremlin would be removing the lifter from the rocker arm. My first impulse would have been to pry out the locking ring at the top of the lifter housing in the rocker arm, but the lifter is actually just tapped (or hammered) out through the bottom, and a new one slides in and snaps into place.
I have immediately noticed a very significant correction to the rough idle/stumbling problems that have plagued me for so long. There is still a very faint roughness, which I attribute to my fuel/air mixture needing adjustment (like a dope, I forgot to do it while I was there) and perhaps a dirty fuel filter. Power is a bit greater, since the “crippled” cylinder is now healed, and feels smoother and more torquey. Also, hot starts are more immediate.
Overall, a good, and very inexpensive repair. (I paid $75, parts and labor. If I did it myself, cost would have been about $10.00 for the lifter and 30-45 minutes to replace.)
Now, on to that darned idler Arm Bushing...