Been a while, but now that everything has settled I wanted to update those interested:
The writ of mandamus was successful - after the defense filed a MTD there was some back and forth between our attorney and theirs, but ultimately the judge did NOT grant the MTD and in fact ruled that a decision needed to be made on our case (the decision on whether or not to grant a K1 visa to my fiancee following her in-person interview many months back). Two days after this ruling we received the long-awaited email from the embassy indicating our AP had finished and it was time to pick up my fiancee's visa.
Fast forward a few weeks and we are now safely together in US and got married about 2 weeks ago (very shortly after we arrived, just to be extra careful!).
I don't envy anyone still sitting in AP, but I hope that our case can at least shed some light / give hope to those who need it.