I know it is rather late to respond. What worked for me working with a *WEMOS D1 mini* and  the *MH-Z19C* was reassigning the Software Pins used *SoftwareSerial co2Serial(D1, D2);*
In that case the second sketch posted works atleast for me. 

The problem might be that the ESP8266 uses the hardware serial lines for connection to the USB-to-Serial bridge (usually a CH340 or CP2102 in most designs) which means you might encounter problems if you want to use the same serial line for more than one device.