Making each piece a smart module seems best. A plain cheap Arduino Nano board has pins that jumpers with a socket instead of pin end slides right onto. Nano is small and easy to install. Look up DuPont Cables, they make wiring easier!
I have no idea why you need a Mega2560. Not just for a button!
If you have clear line of sight to a smart RR-signal, a TV remote can be read as commands.