Especially with the distances involved many of these functions would best be handeled by their own little circuits and not programming.
For Instance, if you replace the button in line 2 with a switch you don’t need to do anything else. Just run power from the supply, through the switch, and to everything it needs to turn on. If you need to have a button there it can still be done with some sort of latching circuit very easily.
Many weight sensors could also be hooked up the same way so long as they act as a closed switch when the weight is applied.
That said, if you find this very hard and really want to make this project about the programming that Arduino can certainly do everything, but I think you will find that it would take more wiring and hardware to do it that way than to try the non-Arduino simplifications I listed here.
Lastly, if I were building this the kill switch would be MANUAL. Code can hang or get locked up, but the kill switch still has to work. Evan an interrupt won’t work if a short causes a low voltage state for the Arduino.