how to get Finder to automatically open a Finder window / or always keep one open?
You can already open a new window if one is not already open by pressing the 


COMMAND-TAB and hold down COMMAND. 

Release TAB, keep holding down COMMAND, 

then press OPTION and hold it down. 

While still holding down OPTION, release COMMAND. Then release OPTION.

As far as doing it automatically you'd probably have to script something. 

Source: @l'L'l