Well I tried all of this,
When playing in mode "return to now marker" the W behaves the same as the Space Bar.
It returns to last starting point, so these is no difference between W and Space bar when playing.
What I had before and what I like, was that the space bar behaves more like a Play/Pause button.
it must be possible, because while playing you can click on the pause symbol with your mouse.
And if you press W after that it returns to last start point. Please try this. I had this in Sonar 2 and 3 also