Thanks for your suggestions!
I did as you asked; there was no change with the crossover pipe removed. I unplugged the ELR connector and, as you suggested, the idle dropped fairly dramatically but it kept running.
I did nothing to the IP but I keep wondering if the fact that I partially unscrewed the ALDA using the bottom fitting has created the problem by causing some internal IP linkage to jam in the wrong position. Obviously, since the ALDA would not pull out of the IP, whatever is attached to the IP end of the rod that bears upon the ALDA moved upward as I unscrewed the ALDA. I have the MB shop manual for the engine but it doesn't really show a picture of the internals of the IP and the precise linkage of the ALDA. There is a diagram but it is not as useful as a picture would be. I guess I'm thinking that some internal linkage was pulled up and is now out of position. But I don't know if that's possible.
And, of course, the problem may be totally unrelated to the ALDA but since I didn't fool with anything else, it seems a likely suspect to me.
I checked the vacuum lines and other than the EGR and ARV lines that I plugged, I'm pretty sure I haven't left any disconnected.
The car starts quickly and idles fine.
I'm a very experienced gas engine mechanic/engine builder but Diesels are new to me and I'm quite discouraged at the moment...