Seems that there is a lot of nextpcb "representatives" in this community. I guess it has nothing to do with their point system.....

It does not protect against short circuits; the relay is a direct passthrough between the battery and the output.
For the PCB , I already mentioned about making one in my last paragraph. This project is just a prototype to validate my idea. But this way, it is more usable than a breadboard design. Thanks for the link.

I would try to rename the variable "select" to something else; it might clash with something already defined earlier. It was not in your original program. Also why using floats everywhere ? All your arithmetics is integer. Your original sketch was using int and long . With floats you will make your sketch a lot less performant; avoid them as much as possible.