In my opinion you're not going to learn anything by paying a professional. Time is money so even if they know their stuff, their not going to spend any time they don't think is neccessary - and that most always includes teaching.
Second. No professional wants to hear about something you read - especially from the internet. So if you're going to work with this guy, then that's what you should do. If based on your knowledge and understanding his diagnosis or work is not what you expect, either/or explain that to him or find someone else.
The alternator may not generate enough power at slow idle rpm to supply the ignition, plus the parking lights and have some left over available for recharge.
13.1 to 13.5 Volts indicates the alternator is producing some power.
It also is possible that alternator is not producing its potential at any rpm. Did he show you what he based his analysis of 60% down on? Perhaps a scope picture?
As far as the parking lights on etc. Are you going to trace this or hand it off?
