I started this project almost a year ago. At that time arduino core for esp was in pre alpha stage so it was logical to use nodemcu. If you use nodemcu the serial port is already reserved for issuing commands to esp so I could not use it for sniffing traffic.

If I started now I wouldn't use arduino...

I would just flash arduino core on esp8266 and be done with it. It would be a lot easier to make.