shift+enter to start a new line directly next.
enter to start a new paragraph for one space between.
Double spacing: extra and <p> get cleaned up.
Tiny MCE Advanced has a checkbox to stop removing extra p and br to keep your spacing. But sometimes for me (i think just in tables) it ends up continually adding more and more spacing on each save. I have to wait until my final revision to clean up the extra spaces and hit save.
I feel you. WYSIWYG is a sure mess when it comes to spacing. Sometimes having to hit backspace on invisible breaks that does nothing visually but cleans up space. And sometimes having to go into HTML mode to bring something back to the previous line because hitting DEL or BACKSPACE never moves it.