Shared the Arduino code in this thread: https://itch.io/jam/100-button-game-jam/topic/140791/code-code-for-the-arduino
I'm not sure what you're asking on the other part of your question. Could you elaborate?
Viewing post in I would like to adapt the controller code for Game Maker
If you're still looking for a general-purpose API for talking to the board, and you've ever talked to a web service before, you might like the web adapter I just published: Buttons Are Cool Gateway Server
The server exposes the button board as a web service (HTTP GET and WebSockets). Most languages have good libraries for making web requests or talking to WebSockets.
Cheers!