Dual heat zones strongly connected vs setback thermostats
Convection in a stairway, in a reasonably tight house, is way overrated.

Given the square footage of wall and ceiling space - I do not see how the stairway makes much difference.

I run a fan 24/7 at the bottom of my stairs, adjust vents to the seasons, and still can not seem to get my system to play nice. I do have a single zone forced air, so it is not a direct comparison, I realize.

Heat loss is proportional to the difference in temperature. And it is nowhere near linear, turning off/down will always be the most energy-efficient.

It is likely to not the same as comfortable for certain, but is true in the case of BTUs