Hi, All this was very useful. I am also undertaking a project to replace the Lennox Smart screen based thermostat with Nest. I am taking a very different approach. I also want to do Zoning. So, I am using Lennox Harmony III ( I was lucky to grab used on on ebay) and I am connecting Nest Thermostats to it.
The good thing with using Harmony is that Harmony Zone controller takes in any standard thermostat.
Ill post my success/failure story