Ok, so I've made some interesting progress.
I have discovered that the whole strip lights up, but when connecting the 5v line from the strip to the 3.3v out on the Pico, and execute the following code:
require("neopixel").write(B15, [255,0,0, 0,255,0, 0,0,255]);
Those pixels go very dim but turn the required colour. The remainder of the lights stay on in a redish colour (which I assume is due to voltage drop but would otherwise I assume, be white).
Its almost like its wired in revers but I have checked the hell out of things so I am fairly confident.
In response to your questions:
1) Tutorial: http://www.espruino.com/Individually+Addressable+LEDs
2) Literally: just working with the line above