1. If you email ADGA, they will let you know if it is taken. Also if you have the membership directory there is a list of herdnames that have been taken, but of course that has not been updated since March I think so it is not completely accurate. If you are thinking of one, I can check quick and see if it is in there for you.
2. Yes, it will. You only have 30 spaces for the name (including spaces). So if you have a long herdname it will take up a lot. Mine is Golden Ears, so I have about 19 spaces left for the name, which works for me. But if you like to give your goats really long names you may want to just use letters for you herdname or something.
3. I am not sure about that. I paid the 15 dollars to register my herdname, and I think as long as you keep your membership dues paid it will stay yours. If you are planning on doing that I do not see any reason to do the permanent thing.