SWEET!!! :) Yes- that file name turned out to be the problem, because after I did exactly what you said, the restore completed successfully (with all the files that I had deleted from my themes directory, put back in place- yea!)
So it's possible that this has never come up before because most site titles aren't as crazy as mine is (and it could also be that, in combination with running on an IIS server- perhaps Apache servers can handle creating directories with names like that. My experience so far with working with sites on IIS servers versus on Apache servers, is that I get confounded by these sorts of problems left & right with IIS...)
So although I do now have a "work-around" for this, and it probably won't come up very often in general with people using your plugin, I was wondering if you would perhaps consider making a new version, that truncates the site title in the file name after so many characters, in the event of of the site title being so long? (And maybe strip out "non-standard" characters, too- it's also a possibility that it wasn't the length of the site title, but rather that it had characters in it like "|" and "+" which also happens to be the case with my title. When I performed your test, I replaced the entire site title with just the word "test"- because I was so eager to see it finally work so I wasn't up for playing around with all the different possibilities...)
Well thanks again, for all your help in getting this resolved!