So, after being sick for a good portion of the month, just got around to fixing this issue. Here's how I (actually my nephew did most of the work) fixed the problem.

1. First, drained all of the old coolant, both from the radiator (screwdriver-style petcock on driver's side bottom) and from the engine block. I couldn't find the engine block drain when underneath the car, but my nephew did. A bit of a pain, because on the OM617 turbo engine, the turbo is "in the way" of getting to the bolt from above, so it has to be gotten at from underneath the car. OR...as luck would have it, you can do this by hand, feeling for it from above...IF (believe it or not) you have a hand-screw petcock that someone at some point installed in the drain block instead! Made things a bit easier. Of note, in order to reach it from above, we had to remove the air cleaner assembly as well.

2. From that point, we drained the engine block coolant. Even though the radiator coolant looked terrific (nuclear green---almost like new), the stuff from the block looked green but had a lot of oil mixed in. That said, believe the block drainage was picking up oil deposits from the bottom of the engine as it was draining.

3. Next, according to the radiator flush I used (details on it later), we were to rinse the entire system with water. Once filled with bottled water, I started it and let it idle for about 15 minutes with the defroster on. Then, that was drained from both radiator and engine block.

4. My nephew's idea next was to backflush the heater core using a garden hose. Here's how he did it, and I think it's a good idea to do so in this manner. He simply disconnected hose #4 from the engine block and #7 from the monovalve (see link below for pic to see which hoses I'm referring to). Using a garden hose connector tip that narrows at the end and is controllable in force by twisting it open or closed, he was able to both front and backflush the core. When first inserted, there we a few burps, then some clear fluid momentarily came out, then some lumpy looking goo came out, followed by the nuclear green coolant and then clear water. After the backflush, flushing back forward yielded clear water, so the heater core flush was now complete.

(Scroll down the webpage below to see the pic of the hoses I'm referring to.)

Coolant Hoses - Mercedes-Benz Forum

4. Now it was time to use our radiator flush. My flush of choice was Blue Devil Radiator Flush, since it claimed to get out "grease, rust, grime or any other build up" and was readily available at a local auto parts store. I would have used Irontite's flush (as recommended here on the forums), but seems I can't find it locally, so...went with the Blue Devil. After emptying it into the reservoir, once again, topped off with bottled water and ran for about 15 minutes. This was then drained from the radiator and engine block.

5. At this point, everything was looking very clear, but...Blue Devil said to run another water flush to get rid of any of its remaining residue. That was done for 15 minutes, then the drain procedure.

6. Finally, was ready to dump in new 50/50 mix. Yeah, I know, should've used the Zerex G-05, but I used what was on hand (the nuclear green stuff, don'tcha know!). We filled it with around 2.5 gallons, then let her idle away for about 15. Problem: not getting any heat! Solution: rev the engine a bit, off and on, wait for the temp. gauge to come up, and viola! HEAT! I should note that somewhere in here, we electrically reconnected the monovalve and aux water pump, as I had disconnected them previously as a test to see if they were the issue.

FINAL THOUGHTS: Would it have been better to drive the car around, especially on the final fill, to get the heat working in a faster manner? Sure. I'd also say driving around is the way to go in order to "burp" the system---it's possible the "no heat" issue after final fill was caused by air in the system. That said, I don't see the need to do anything special to "burp" the system; your mileage may vary on that. I also believe that, should your system be really dirty, driving around is much better to help the fluid circulate more to clean it out. Also, I have not totally ruled out the aux water pump as faulty; the car does turn out the heat at idle now, but with less force than when the car is moving. It'll get replaced at some point.