Sounds like a great setup you have there 
As you can see in the items page (lines 4,5,6 and 17 as of commit c550516)
The relays status and temperature setpoints are accessible via MQTT (mosquito) for both read and write so simply adding these MQTT items in your existing installation will be the only thing needed to remotely control any HestiaPi installation in you network.
So an item like this would do.
Number TempSetpoint "Temperature Downstairs Setpoint[%.1f °C]" { mqtt="<[mosquitto:hestiadownstairs/local/settempsetpoint:state:default]"}
Notice the different mosquitto.clientId hestiadownstairs which you will need to modify in each
/etc/openhab2/services/mqtt.cfg
Sure they will. Leaving relays and actual controls aside, you can test this yourself by running a HestiaPi on any RasPi without our custom PCB (just flash our image on an SD and boot it) and publishing to the desired MQTT topic from your laptop while watching the web/app interface of that RasPi from your phone. It is also quite fast on RasPi Zero (<1sec).
As the MQTT topics are for both read and write you can even turn off the underfloor heating from upstairs if you forgot it on before going to bed
I cannot know for sure but if this wiring centre controls these valves with some logic, I don’t see why you cannot use the 3-relay Hestia’s relays to control each one separately
Let me know if you get stuck!