First off Welcome to SteelSoldiers and MV ownership.
Second, this tread is a couple of years old and covers the M1010 CUCV. It is a totally different animal than your M1009 CUCV. Look elsewhere for answers.
You need to visit the CUCV Forum and read the following:
At the top of the page is the CUCV Wiki
Cucv Wiki - Steel Soldiers::Military Vehicles Supersite
It has alot of infomation and links to all the CUCV Tech Manuals. You will find the wiring diagrams in the Appendix of the -20 and -34 manual.
And then read the FAQ Thread
http://www.steelsoldiers.com/cucv/20600-faq-thread-cucvs.html
It has alot of info about parts, trick, tips, etc......and alot of junk....
Now to your questions.
The Idiot light for the alternators MUST be in place and working for the alternators to work. If they are lit up then the alternators are not charging properly.
In a stock system, each alternator, when functioning, will put out ~14.5v.
When GM designed the alternators, they modified the 27SI, type 100 alternator ('80s Cadillac) to have the internal electricals to NOT use the case as a ground. They instead "isolated" the internals with insulators and added a "ground post". This allowed them to take the 14v from the drivers side alternator, feed it to the passenger side "ground terminal" and have 28v at the passenger side positive terminal. The alternator is only generating and additional 14v but combines it for 28v. Sounds confussing but is really a very simple(ingenious) design.
These trucks have lead a very harsh life. The electrical contacts have corroded and dirt/gunk built up over the 25+ years. By cleaning the fuse block, ground connections, etc. 95+% of all electrical issues can be solved. You should also remove the instrument cluster and clean the contacts and check all the bulbs.
When I an working on an electrical issue I will cut and paste the particular wiring diagram and "color" the wires that I am tracing. It makes it very simple to "see" the circuit.
Also look at the first few pages of the Troubleshooting Section of the -20 or -34 manuals. It covers how GM designed the wiring diagrams and what the wire numbers represent.
Good luck and God Speed.