Regarding use of 'so', in a throwaway manner, surely this is just a matter of style, no more or less.
Am reminded of the Vonnegut usage: 'so it goes'. Very effective it is. Or, 'so it is' - a form of phrasing heard a lot in the Irish dialect and I find myself saying this - it sounds better somehow. Or I may just be mimicking the Irish OH, which is how it spreads, so it does.