This is a bit of a stretch but can an enemy LS unit enhanced by a Death Mark be affected by the "Discard all Black Sun units you control" text on Show of Force. It is obviously not a unit you control and probably not a Black Sun unit (unless it was turned) but does Death Mark make it qualify for the effect? Or is that considered just the cost of playing Show of Force and not a targeting effect at all?

It's not a targeting effect at all (though not actually a cost). For an effect to target, it must use the word "target."