I willl add holes at the approximate positions indicated and you can correct them if they are not in the right place.
Yes I can make a new sketch for the arduino, the pressure sensor and the buttons with the connectors to the boards from the arduino indicated. I’m not sure what a “or can be a small calculator, a screen” is.
Probably not, but you are a long way from the supply and a bypass capacitor is wise. With the pads on the board you don’t have to mount one to start with but if you have problems later you have the space to mount one. If you don’t install the capacitor you also don’t need the ground wire which reduces the wire count by 1.
That should work fine. The pressure sensor is I2C (i.e. the output is digital not analog) so having the mega some distance away shouldn’t be a problem.
The trade off here is reliability. With multi pin headers you can bolt a plastic plate on top of the connectors to hold them in position. With individual connections I expect that will be more difficult. The connectors in the accordian will be moving around and will likely tend to come loose and disconnect if not secured down. That said. which ever way works best for you is fine, individual connections will work the same way as muilti unit ones and you can change one to the other later if needed.
OK, the next set, first a new one with the mega, barometer and switches schematic only:
edit: I replaced this with one that also has the midi connector installed (although I think it needs a ground somewhere to function) which I missed before.
accordian-mega.fzz (33.9 KB)
then the right and left boards with the mounting holes and your length changes (which I’m not entirely sure about):
accordion-left-hand-Sketch.fzz (176.9 KB)
accordion-right-hand-Sketch.fzz (184.5 KB)