bag is the smallinventory, so that does differ.
you may make a shortcut for your questwindow if you wish and then assign a key, just check the /show in the help window for explanation how to make shortcuts for windows.
the standard shortcuts are supposed to work for all supported operating systems, therefore they have to work on a single button mouse as well - additionally some people prefer to use shortcut keys instead of the context menu, e.g. me

though having buy listed twice indeed does not seem to make sense - feel free to file a bug in the
bugtracker about it.