1) Head width-- Cornhead is simple, but what about beanhead going diagonally in 15" row beans. What width should I put in for a 925 since I know I don't cut a completely "full" 25'?
Put in 24' to start with and compare your recorded acres to you "boundry" acres. This will be a good indicator of how well you drive the combine - however - you are combining diagonally and you will have "empty" acres being recorded every time you start and stop a swath!

2) Do I use the Greenstar on a load or two, then weigh the loads and somehow calibrate it to match the scale weight?

Calibrate the combine without the GPS - the owners manual actually has pretty good instructions on that. 
3) Assuming I calibrate the weight somehow (see question 2), must I do it again for each different crop?

Ideally you would calibrate for each crop - but in yield monitor situations I presume you are putting your crop over a scale at some point and time and your crop totals can come from there to compare to your combine totals. Everybody has a different need for accuracy. I have seen people calibrate almost daily with their grain carts (normally nothing really changes) and I have seen some people calibrate once every year or two! 
4) I know NOTHING about precision ag, not even enough to ask the right questions. Any help for a beginner using Greenstar would be GREATLY appreciated.

Have you tried to talk to your local dealer yet - quite a few of them have somebody on staff that could help you (I am that support person at AgraTurf Equipment Services). As the AMS consultant I really don't care where you bought the combine - my job is to support, demo and sell GPS. If I was your local dealer, I would help you with question #5 and give you a demo!   
5) What does it take to have mapping in addition to yield monitoring? I'm sure I would need GPS, and additional hardware. What would I need and how much $$ would it take?

Paul VDB

Woodstock, Ontario