Hi - I did go to Windows update and did a "check for updates", but the only results returned were two updates for the newly installed I.E. 11 (which also works and allows uploads without locking up, by the way). I downloaded and installed those, but I saw nothing there for Office. I verified that checking for updates to all MS software is selected and enabled. If I had found an Office update, I wouldn't have allowed it to download, after all this.
At this point, everything seems to be working as a result of reinstalling Firefox and I.E. - among the unexplained questions that remain is, if it actually wasn't related in some manner to 360, then why did it work faithfully each time I tested it with 360 removed (normal boot mode)? If it had been malicious in nature, wouldn't I have received a warning message when 360 was intact, rather than the browser freezing up with no explanation? And also, if the Office update wasn't legit, why didn't 360 intervene when I ran it yesterday morning? At that time, 360 was running, with no errors reported, and Live Update set to automatically update in the background.
I'm glad that I got it running properly now...in fact, better than before, as a result of all the registry cleanup I did while troubleshooting, I'd wager...but I'd be interested in learning exactly what did occur, and if there are preventative steps I can take to prevent this frustrating scenario from repeating again.
Next step is to use the laptop normally, and run it through its paces a bit, in order to verify that there aren't any issues I haven't uncovered yet. If all seems well, I'm going to image the drive to have a reliable backup to turn to in the event Murphy's Law isn't quite finished with me yet.