For example, if I have a 160x600 ad on my desktop version, does Google render a 320x50 mobile ad if that is the CSS height/width I provide for the responsive Smartphone version?
Yes, it works. Just like DFP, Adsense renders the correct size based on element's dimension. But you have to remove inline styling and set it via css. When the ad unit has an inline styling, css overriding doesn't work.
BTW, I've no idea if Google will let us use it in this way.