@mesprit It's just the way to make things easier. On barely every mashine BASIC will show Hello World with
10 print "Hello World"
The CPU instructions for doing this are different so you might need a different BASIC interpreter for every "system".
But this is also very abstractive - but hope you get it.
Imho you can use a Pi0/1 image on a Pi2/3 revision but you will loose benefits for some optimized emulators. That's what @mitu said in his previous posting.
You can use the RetroArch network interface to send commands to RA through a script that's called when you press the button - http://docs.libretro.com/development/retroarch/network-control-interface/#enabling. It looks like the interface supports the 'Disk Next/Prev' commands so you can switch disks, but I don't see a command to get the current disk index. Maybe you parse the RA log to get the current disk index and determine if you need to go next/prev in the disk index list.