1. Because someone else had already used it.
3. It won't. It will prevent others from using your account. To avoid losing names, log on your chars more than once a year.
4. Then maybe your main char somehow got skipped. I don't have a good explanation for that but I think I have heard about similar cases before.
5.
http://forum.warmane.com/showthread.php?t=372953
Premium accounts were not excluded from that.
If you want help, make a support ticket on website
here. But they usually don't hand out free name changes so I don't know how they could help you.
Which part still makes you think that I don't know what happened?