My mk_arcade is edited (by me) specifically for use with the Freeplay Zero/CM3. The code that RecalBox uses is different. It does not handle the negative number. In my code, the negative number means that the logic is reversed. Most of the buttons go low (to ground/GND) when pressed. The power button is different (because of its usage for booting the system). It goes to VCC (applies power) when pressed.
The base mk_arcade code assumes that all the GPIO buttons are high until they are pressed and then they go low (GND). My version of the code can be found at
https://github.com/TheFlav/mk_arcade_joystick_rpi
It also (as of recently) has support for analog input (like PSP1000 sticks) using a specific A2D converter.
My version also has the option to treat the hotkey as a toggle.
The typical way is to just have a bunch of buttons and then (in RetroPie/Recalbox) define one of them as a hotkey. This button, when held, puts the system in hotkey mode so that you can press another button (at the same time) to get hotkey functionality.
My version allows you to define a hotkey toggle mode where the hotkey is pressed-and-released to put the driver into hotkey mode. Once in hotkey mode, the next button that is pressed gets put together with the hotkey button to be presented as a button combo. I know it's a bit confusing, but it was done to allow the power button to have this extra functionality that doesn't conflict with the power-down functionality of holding the power button.