First off welcome to the site.
Long stories are OK, in fact we like stories and love pics. But in the future break them up into paragraph's for ease of reading and you will get more responses to your questions.
Having a lower rated alternator then what is required is a problem, but might not be all your problem, or could be. That should be the first order of business you take care of. Here is a direct bolt in upgrade to a
95 amp alternator done by one of our members. (if you don't know, the green wording is a link that will open the page )
This should be your first course of action. I know you bought a new one, but it is under powered for your truck. Once you do that and it does not solve the issue, we have in out tech section all the manuals and wiring diagrams for our trucks, and a whole lot of members with an extensive background in these trucks that will be more than happy to help you out.
My one concern is when your brother took off the + cable and your volt meter showed the correct voltage, my concern is that you might have a short in 1 or more of your fuse-able links or wiring that is on the positive cable circuits. But like I said, take care of the alternator first to eliminate that problem out of the equation.