This is a great site to help you get a better picture of how things work. Overthinking things can indeed be a problem. Often, fixing the obvious first solves lots of mysterious issues. I have been doing this for 40 years and know batteries are just rented and never really bought. When in doubt, throw them out. Messing about with things one is not sure of often installs more problems by blowing fuses, not replacing wires, and even worse.
This has been a great learning experience for you and others. These are complex machines and no two are alike.