Well, following proper etiquette, I'm here to post the conclusion. I should also point out that this thread is actually related to this thread:

143A alternator from 96 S420 in a 96 E300?

Because cleaning up the ground straps was part of the alternator check/replacement job.

I didn't do anything to the last ground strap (to the right of the big ground strap from the bell housing in the picture posted earlier) because when I put my ratchet on it and "grazed" the anti-roll bar- there was a spark, which startled the beezeejus outta me. I went online and tried to research this strap- but out of time, patience, and money and after the nagging from the other room, I decided to just let it go and finish what I had started.

The cause of the belt slip was greasy pulleys and serpentine belt. The last two oil changes were done with filters I provided, but the filter cartridges I got through "special order" didn't come with the large o-ring for the filter housing cover. As a result, the o-ring that was in there lived through 2.5 oil changes- at which time it, oil began to seep around the cover. I guess the oil was flung around, and ended up mucking up the pulleys and the belt. So- about an hour, a half can of carb cleaner, a roll of paper towels, a 2" wide brush from the dollar general store, an old athletic sock (which actually fit into the pulley grooves nicely), another hour and a new serpentine belt later, no belt squeal. Fortunately, it was (a freakish) 55 degrees here today- so I was able to open the garage doors- so I didn't have to inhale too much carb cleaner fumes and get dain bramage.

The result? When I revved to 2500 rpm and watched the climate control display to see if there was a change in voltage- I was shocked because the dome lights burned brighter than they have ever been since we owned the car, and the voltage jumped up to 13.5V. This has to be due in large part to replacing the rotten green asparagus ground strap, and cleaning up all of the brown wire- ground points under the hood, etc.

So- all done. Thanks for all the helpful posts! If this thread helped anybody out- post here let me know! Cheers- J