Just a quick follow-up from my last summer update. We had the HVAC contractor come back for some assessment / adjustments and in the process marked the existing positions of dampers and vents prior to adjustment. That way when winter comes rolling around we can just change the settings back to what seemed to work after our first go-around without any fiddling around or trouble. After some adjustments to the main dampers I'm proud to say that our summer second floor heat wave seems to have been mitigated. Granted the HVAC unit runs a little longer since the first floor takes a bit longer to cool, but now everything seems consistent, which is what we wanted all along.
Hopefully this is a sign that things are finally settling out in the right direction!