A brief update to follow-up: Since my 12/2 post, the glow plug dash light in my E300D has started working normally again without my doing anything, so I have not tested any of the suggestions provided. I will say that there seemed to be no apparent difference between startups when the glow plug light was working and when it was not. In each case, the car started easily, without vibration or roughness, and without the presence of any exhaust smoke. Should the dash bulb stop working again, I will run through some of the suggestions and report back to the forum.

Thanks to all who responded to my questions. Sorry I could not provide a more definitive response.