Questions regarding : WS2812B + ESP8266 + External PSU + Arduino Uno R3

Maybe, yes, using a usb-serial adaptor board, but it will be a PITA to use even if you can make it work. I would replace the NODEMCU with a wemos mini, but it must be a genuine one from Ali express. There are many poor quality clones, including on Ali express. You must buy from the Lolin / Wemos shop. I have never had any problems with the genuine ones. But I have a few clones, and the micro-usb connector broke on at least 2 of those.