Help wiring a Nest thermostat in an unusual situation
You're a bit stuck here

The Carrier Infinity system isn't a standard "on-off" control, unlike conventional thermostats.  Instead, it uses a serial connection between the thermostat and the controlled devices (zone controller + furnace in your case), which the Nest hasn't a clue how to speak.

Your best bet for integrating more sophisticated automation into your system would be to use some sort of gadget with a RS-485 interface to speak the Carrier Infinity protocol.