I don't use Calibre, but they're not DRM and manually unpacking the contents with MobiUnpack works for me and yields HTML files which my browser is happy to open and KindleGen pleased enough to re-mobify (albeit visibly messing up the paragraph indents and who knows what else upon reconstruction).
Good to hear that Smashwords seems to have restored their deleted-from-catalogue downloads, though I remember this book having such annoying sudden font changes between paragraphs as to render it unreadable, which is why I didn't pick up any sale Slonczewski last year because I couldn't determine whether I liked her writing or not based upon it.
ETA: My ghod, there's a lot of cruft in these. Why would you set font face tags for random segments of every sentence throughout the book? What on earth are they using to generate their Mobi files?!
At the very least PPP should strip all that out so that they don't get dinged as much for the per-filesize delivery charges Amazon charges under KDP.
Maybe that's what Calibre is choking on.
And this is another reason why I don't consider mobi-only format files worth paying for. At least messed-up ePubs are easier to fix, most of the time.
ETA 2: Just doing a simple search/replace to blank remove the font-face tags alone managed to save me half the file size in the HTML version.
And I haven't even started on their weird use of empty divs with height set for who knows what reason (apparently to provide those annoying blank spaces between paragraphs) and the p tags with height, width, and totally extraneous justify set for each and every single one.
ETA 3: Downloaded filesize: 2.1 MB. Cruft removed and re-mobified without any compression: 1.4 MB. On -c2 tightest compression setting: 1.1 MB. Difference in delivery costs deducted from royalties on the 70% royalty plan if used: 15 cents per sale. Joke that I could be making here: priceless.