It occurred to me that I have a second MacBook that I could try to install it from, so I tried it with that, and it worked!
I had to update my Xcode software -- actually deleted and then installed new rather than update through the Mac App Store -- but before that, I had to upgrade [finally] from Yosemite to El Capitan on this older 2009 MacBook I was using. A silly amount of work for a novelty when I don't even have a controller, but it's about time I updated this OSX, anyway…
So if you have a different Mac that you can try and install it from, you could always try that. I really don't know what the difference could have been. This one gave an error for TopShelf as well (might've been a different error?), but it fixed it this time and installed as it should.
But basically -- I installed Xcode *new* on my 2013 MBAir yesterday and Provenance/TopShelf gave me trouble. I installed Xcode new today (deleted current install, upgraded my OS, then installed Xcode fresh) on my 2009 MBPro and it worked just fine.