The Mac OS X 10.9 Mavericks download script is now hosted on Mavericks Forever. To download Mavericks, open a Terminal on any Mac and run:
curl mavericksforever.com/get.sh | sh
Or, navigate directly to mavericksforever.com/get.sh to read what the script does.
Odd, I'm not immediately sure what to do about that. (I know you got it working but it's a shame it wasn't a smoother experience.)
If I recall correctly, the InstallESD.dmg inside the App Store's Mavericks installer was never restorable, even when Mavericks was current. But of course createinstallmedia worked at one point.
From what I understand, Apple updated the App Store Mavericks installer some years back to re-sign a certificate or something, but ended up breaking it. Or something like that, haven't looked into this closely—but It's not only broken for you!