"If the 'Growler' (love the name) system was in use, I suppose a way to defeat it would be to saturate it with more cruise missiles and decoys than they have launchers??"
they won't deploy a single S400 without some additional defenses... and they could easily recognized flight path of cruise missiles and take those with a lot cheaper AA than S400
actually I doubt they would bother with that, Putin wouldn't be reluctant to use nukes in that case, tactical at least