I have a 99 ML 320 that I've owned since new. I get CD loading errors but only in really cold weather. I just learned to live with it. First time I stop, I pull the cassette out of the changer, spin the disks a little bit with my finger, and re-load it.
I have no info on your second question.
Ditto on both. 99 ML430.
Sometimes certain discs work, sometimes they do not. All of my discs are burned. I find compilation discs especially problematic.