Newbie Questions

I am considering this system - I have a 3-story home with a huge temperature differential between the floors. The layout of the ducting precludes a conventional zone control system as there is a large inaccessible duct running between the floors that feeds the ground and major part of the second floor. So this Flair system seems like it may work by controlling locally at the room regardless of the duct layout. 

The first question however is the pressure limitation - 1/3 of the vents only being allowed to close is going to be an issue for my system; I pretty much don't need (want!) any vents open on the ground floor during summer months and that accounts for almost half the vents! I'm not worried about over-pressure, these are all typically closed during the summer months anyway. (A work-around - although I'm sure not at all endorsed, would be to just to enter that there are more vents than actual in the initial set-up, to effectively create your own limit %) - I'd also not be averse to installing a bypass duct - it would be nice if Flair gave us that option to implement that and over-ride the 1/3rd limitation completely. 

Next - I'm unclear how the individual room, makes the Main Thermostat call for Heat (or Cooling); I have yet to choose a Thermostat system, was thinking Honeywell T9; Manual Room selection is easy enough, but with full Auto, I'm not sure how it will enable the air system to come on.

Here's where my lack of comprehension is coming from - with a true Zone controller, each Thermostat is connected directly to the Controller. And that Zone can not only control its damper(s), but also call directly to the System to send heat or cooling, (prioritizing whichever zone calls first) 

Using Flair and 'some' Smart Thermostat (I'm happy for any respondent to pick a brand as to how it works with that one) - let's say top floor is 78, main Floor is 74 (where primary Thermostat wired to system is located) and ground floor is 70 - I want the Top Floor to cool to 74 but don't want the ground to get any colder. The 'Average' temp is already 74 and I would want all ground floor and 2nd floor to be closed while the top calls for Cooling and the registers on the ground and second floor all close (which I know cannot happen because of the pressure limiting component) 

So how exactly would this work in a fully automated scenario? 
 
The Smart Thermostats with remote sensors are typically (are they?) working to achieve an 'average' - does it work differently somehow with the Flair system?