Pico's ESP8266 weirdnesses (Module not found)

trying to flash the pico via the CLI instead of using the WebIDE (always resulted in several false values during verify)

Did you add the -k switch? http://www.espruino.com/Serial+Bootloader

You'd need the stm32loader.py from the Espruino GitHub repo. Basically the F1 chip doesn't have a USB bootloader, so I made one that emulated its' Serial bootloader - however an errata on the STM32F1 chip meant that USB was unreliable unless you changed a few registers first (which that -k flag does). The Web IDE does it automatically...