They are automatically added if they are in the mapcycle.txt file or whatever the mapcyclefile cvar points to, you should also read about {cms_selflink page='70' text='votemaps.ini'} and {cms_selflink page='67' text='adminmaps'}
Yes offcourse, read more about it here
No. Short version is BAT needs the steamid to validate into something. Longer version is that BAT does not consider a player fully conneted untill he has a proper steam id, and him never doing this will create problems. There is also the side issue with that fact that when players STEAM ID dont validate their most likely on a hacked/cracked steam account or perhaps the server your trying to host is cracked (Its offcourse posible with a bug in either BAT or steam somewhere).