Adding common wires for nest thermostats
First, if the Nest isn't reporting problems from lack of a C wire, then you don't have problems, so don't make problems. 

If you do need a common wire, I gather by the grand tour of your system that you have no idea where C can be found.  The answer is the transformer has 2 terminals on the Low voltage side. They are  R and C.  

Hypothetically, if you ran a C wire up to your thermostat and accidentally attached it to the R wire on the transformer, absolutely nothing would happen.  (The Nest would be trying to power itself off the voltage difference between R and R, which is 0 volts.)

You see where this gets pretty easy.  

You can also tap "C" at any place it goes, e.g. Any solenoid, relay or actuator.