Ryan Homes fixer-types came yesterday and took care of the upside-down joist. Rather than trying to replace it, they opened up the ceiling in the basement and installed an extra joist next to the existing one. After a bit of chocking and shimming, they managed to get rid of most of the creaky bits of the resilient flooring and it no longer has any spots where it bows when you tread on it.
At the same time, a few kitchen cabinet doors were replaced (we had some that rattled) and the plumbing issue involving the guest bathroom was resolved.
We still have a few outstanding issues (including drywall repairs from the work done yesterday), so I will report back on those when they are completed.