An update: the new stem FINALLY showed up, and my husband put the Bianchi together with the set-back seatpost, a new Fizik Vitesse saddle (it had the most fore-aft adjustability of any women's saddle we found, and it seems to be a good fit for me all around), and the super-short stem. Five centimeters. It looks ridiculous but it did the trick. We've got KOPS, we've got a ninety-degree angle at my shoulders when I'm on the hoods, I can reach the drops. It's far from perfect -- five centimeters is a crazy short stem, and the front hub is not obscured when I'm in the drops, but it is very rideable.

The steering is a little squirrely when I'm up on the top bar, but not noticeable on the hoods. I think it's just completely compensated for by the fact that I can actually reach the hoods without stretching.

I will get a new bike eventually (and give this one to my husband) but this will work for now. Thanks for all your help, everyone.