Matteo, you sound like a cool and very bright young man. I have a copy of a great trouble shooting guide (from my son's BMW factory repair manual) that I think you will find very helpful. You think your multi-meter is handy now; just wait until you find out how to really use it! Yes, it takes some effort but has clear diagrams and good explanations. Would you send me a PM with your email address? I'll send it the next time I am in my office at work.
Also, Vaseline is very good to prevent corrosion on grounds. Most of the time, electrical issues are caused by corrosion on ground contact points. Look at photograph #2 in your second post; it looks like a potential cause for the issues you are experiencing.