1.2 and 1.4 is the official supplied by ClockworkPi. JCS made a custom one which uses the chip completely differently but is compatible as far as the Pico is concerned.
Yes, they all get uploaded to the same chip in the PicoCalc motherboard (NOT the Pico)
“Releases” at the top of the page… Making sure you're not a bot!
i found the steps in the readme for JCS’s firmware to be as step by step as possible, are those not clear enough? Making sure you're not a bot!
also, note that those steps can be used with any BIN file made for the keyboard chip, not only JCS. so you can follow those instructions with the official BIN too