Well, it's fixed. It has been for a couple weeks now. I just haven't bothered to write about it because I know y'all are tired of hearing me bitch.
As expected, the entire top case was replaced. At minimum that included the keyboard, trackpad, and battery. I suspect that the system board was also new, since the SSD (hard wired to said board) was returned with a clean install of Sierra (unfortunately not High Sierra as it had gone out with).
The keyboard (as evidenced by the new markings on the control and option keys) is from the mid-2017 line, even though my original was a late-2016 model, leading me to believe that the entire machine was swapped out except for the bottom cover (where the serial number is engraved) and that the serial number itself was transferred from one machine to another in firmware. I understand there are some minor internal changes in the 2017 keyboard design (there have been far fewer complaints of keyboard issues), so hopefully this will be the last trouble I have with this laptop for however long I end up keeping it.