Search the forums for System Link there is a really good post with a power shell script that can set up wow copies for you with out copying the 14-15gb the game now is.

As for the accounts, here is the flow.

You log in with your bnet account, if you only have one WOW account it just logs you in, nothing extra to do. The first time you log in to a bnet account with multiple wow's you will get a dialog box asking which sub account you want to use. From there on out, underneath the Username And Password boxes is a drop down list with your wow account names in them. It -might- reset if you do not check 'remember username' or whatever it is.

I have never had any problems logging into WoW because of the bnet account. [shrug] with however many millions of people playing WoW now its not surprising that you are going to find Someone that something bad has happened to in any area.

As for the drop down menu its on the log in screen and will appear the first time you log in with a bnet account that has more than one game of wow