Soooo... everyone's probably all a-flutter, wondering how this all turned out. I really can't thank you all enough for pitching in with suggestions so quickly... and not a single "what an idiot" comment!!! What kind of forum is this, anyway? Have I stumbled onto the anti-internet?!?!
After a late, lengthy evening enjoying a quantum of solace and a good night's sleep, I was a bit more relaxed and resigned to the situation; if the diff had to come out, so be it, but I was still hoping it wouldn't. I had enlisted the aid of a good friend of mine who is an absolute ace mechanic; he's the guy the local Mercedes and BMW dealerships call when they can't figure stuff out. He brought over a borescope and pro-mechanic-approved milspec rubberized tablet, neodynumnenckryptonite magnet welded on the end of a flexible cable, flexible grabby thing... and I had the usual assortment of picks, mirrors, and a good intense LED flashlight. Between his expertise and my prayer, it was sure to come out.
An hour later, it was still in there and proclaimed immovable.. and maybe not even identified.. by a pro. Tow it to his shop on Friday, he'll pull the diff out, open it, clean it up, and reassemble it. He's a great friend, but I'd still pay him what the job is worth even though he'd probably argue with me. Plus, he's always way backed up and he'd probably have the car at least a few weeks. I was bummed, resigned, but relaxed.
After he left, I figured I might as well give it some more effort on my own.. .what did I have to lose? Soooo... hibeam LED flashlight in my teeth, mirror in RH, paper clip in LH, under the car I went. Got the mirror at just the right angle.. saw a nice clean silver bolt head that I had THOUGHT was the magnet by feel last night, and had been attracting the paper clip... but only because the magnet was tucked a bit farther in, NEXT to it. I could SEE it!!! I had tried this yesterday, but only with a general lighting LED wand.. not a spot flashlight. THe much better view with the pinpoint spot finally showed me where the magnet was. Tried a bunch of different bends in the paper clip, and what eventually allowed me to dislodge it and work it towards the hole was the "get behind it/lasso" technique some of you mentioned. Once it was dislodged, I slowly worked it towards the fill hole (I still can NOT translate the mirror image in the mirror to the correct motions and directions necessary.. I kept going opposite directions), where it almost self-oriented longitudinally for extraction.. just had to nudge it away from the edge and towards the center of the fill hole. Once there, I was able to grasp it with a pair of extremely slim dental forceps or whatever they're called. Slipped off a couple times, but each time it was a little closer.. until... finally... I had accomplished the impossible. Thank you all, thanks to my friend who showed me how to use a mechanic's mirror properly, and thanks to God... no way this would have happened without any of those.
Bought a new magnetic fill plug, Filled the diff w/ el cheapo 75w90, went for a 10 mile warm up drive, drained, and refilled w/ Royal Purple. Almost no visible residue on the magnet after draining the "flush fill," but better safe than sorry.
Sooooo... a huge thank you. One of these days, I'll learn to pull a diff and rebuild one. Thankfully, TODAY IS NOT THE DAY!!!!
Here's two pics.. one of the offender, showing the chewed off section; the part that is missing was in pieces, but at least visually all the pieces seemed to be still stuck to it magnetically when it came out. The second shows the tools that did the trick, with special honors going to the paper clip on the far right.
Rock thee hard, yon MUDsters.
