ReverseEMF:
When you say "works" are you claiming conformance with all specifications, or simply that it empirically "works"?I, personally, don't know, because I'm unfinilarier with the "WeMOS D1 Mini series". But, I do have experience with the ESP8266 and with the WS2812B.
It works as in it gives me a nice sunrise when I send the correct message via MQTT 
With the level converter nothing happens. (well, the nodeMCU works and is connected MQTT, the LED's don't do anything though)
Isn't there a simple setup on the Arduino Uno that I can try to see if I can get just get the level converter to do it's thing? Forget about the leds for a second to cut timing from the equation (something like my original image)