Looks like someone was kind enough to update the direct download page:
Xenonauts.dmg (V13.01)
Considering I spent 3 hours last night trying to get Desura to complete the download, I'm going the direct route now, which is dumb, but at least it seems to be working.
It's not the latest release, but at least it's something that doesn't involve me babysitting a download that should just work.
Desura is crap and I'll never buy anything through them again.