If you can find them, then I'd absolutely suggest that you try them...I've simply never seen speakers of this specification advertised.
As to the door speakers, yess indeed they simply unbolt from their enclosures. I don't exactly know how these speakers are crossed-over (or at what freq), but I'd think a full range driver of similar size (and resistance, of course) would work fine.
As to the speakers in the rear, I believe the 2 tweets simply do the same job, they are simply pointed in disparate directions to spread the high end out a bit, perhaps with the idea of widening the sound stage by directing high-frequencies in many directions. The rear speakers I do believe are passively crossed locally, meaning that the speaker wire leading to the speaker is running full range.
I hope this helps and wish you luck...I've just bought a '92 300D for work, and dread the thought of re-doing yet another W124 stereo! It'll be great if you can find good speakers to do this...keep us all posted
