Were they vaccinated for tetanus when they were banded? The stiffness sounds like the early signs of tetanus. How is he carrying his tail? If it is up that is another indication of tetanus. Get some tetanus antitoxin and give it ASAP. Also some penicillin. Don't wait!! If the antitoxin is...
All I can say is it varies. It is easier to buy kids than adult milkers, but if you can find a milker for sale, you know what you are getting. With kids it's an educated guess.
The birthing process is usually straight forward with some notable exceptions. You can make anything you want with the milk. You can cook with it just like cow milk. If you are ambitious you can even make cheeses.
I have not found that to be true at all. I always offered mine both a salt block and a loose mineral. I used an iodized salt block. I noticed they would go long periods without touching the mineral and then there were times they ate a lot of it. You don't want them to ingest more mineral...
Goats are browsers. It is certainly OK to let them browse. When they are milking I would think you would want to offer them some alfalfa and give them grain when you milk them. The condition of your goats is the best indication of what they need.
Yup. Had a single goat once. Not a good idea. We called her Bouncing Genevieve because she would bounce over any fence. Once she got a companion she more or less stayed put.
If I was trying to provide milk for nine people I would get a full sized goat, not a Nigerian, but that's me. You can expect a full size doe to produce at least a gallon of milk a day. I have no idea hwo much Nigerians produce.