Suddenly, no AV/malware program I tried will complete

Understood, Moxie. I just wanted to remove anything that even remotely suggested trying to save my computer from anything bad. :) The moment MSE completes, or not, Malwarebytes will be reinstalled.

Hung up in a little over an hour.

Here's Disk management and what the Defragmenter indicated when I first launched it.

We may have a winner. Some more digging on the Microsoft forums yielded at least one other person who seems to have had the same thing happen recently. Apparently it is a known problem, but Microsoft's patch is not tagged as a required update! Windows update / patch KB3092627 fixes the problem, assuming it's what I have. It apparently might be, as after I applied it I got through a Full Scan with no issues (it passed as I expected). I'm going to guess something in the Updates that I installed on 12/8 messed up MSE and this patch fixes it. I don't want to speak too soon. It worked once; I'm going to wait until my normal weekend scheduled scan. If that passes I'll mark this as solved.

That patch did the trick. My scheduled scan ran fine with no hang up. I'm marking this one as solved. Thanks for all the help. As none of the scans I did run identified anything after this was observed, I'm chalking this one up to some breakage that occurred likely from the last set of Windows Updates I had installed.