Hey man, no nothing worked, dont know why it didnt let me "enter" any key. It just didnt do anything an no no key was stuck or sticky ;)
Anyway, thanks to the help of Mr sleve_mcdichael i managed to do it. Yay :D
Thanks for you as well.
From what is advertised the kit should work.
If the "Zero-Delay" encoders (can not judge if they are genuine or clones) put you off (e.g. being not so zero-delay) you can also run your setup with two MCP23017 (GPIO extender) and via GPIO.
If I would build your setup I would provide the left/right flipper a dedicated pin.
With the kit, as well as with MCP23017 you have 16 switches. Usually 4 are needed for the stick axes, and 9 for the most common push button functions: A, B, X, Y, TL, TR, Select, Start, Hotkey. So you have in any case three spare switches.
The third could be "Tilt" triggered by a gyroscopic sensor. (sometimes Space-key on keyboard based pinball games)
Mapping of the buttons can be done on a per game basis for many emulators -out-of-the-box- (retroarch-based and amiberry). And for any other emulator this can be done via runcommand-onstart.sh / runcommand-end.sh  in combination for example with xboxdrv.
Sorry, a lot to digest for a new joiner. But no worries, I assume you will master it by time.
You purposely want to build a cab that can not play most games? 😯
Anyways you'll have to check the keys n the mame menu, i think by default you access that by holding start. you can also open it from the retroarch menu, you can not see it from within emulationstation like you're trying to do on the picture.
Start the game first.
copy the default RA-controller-config to another place of your choice (i.e.: "/home/AMIBERRY Inputs/")
edit the button-mapping as you like in this copied file
go to "paths", navigate the "inputs"-path to your new one.
click on the button "rescan paths"
There you go, the inputs are now as you like to have them global for all games ;)
Sorry, but you're using a 3rd party image bundled with the hardware, which we don't/can't support.
Note that selling RetroPie is against the project's license - https://retropie.org.uk/about/legal/, so you've been scammed, since RetroPie is free/open source software.