You are very right on these points!  Jesus is calling us beyond ourselves, and any lines we might draw that say "this is sufficient".  Clearly, Deuteronomy 24:1 was a line that enabled relationships to stop short of self-emptying love.

Used as a rule -- and enforcement by refusing communion is such a use -- indissolubility becomes another line rather than a path toward self-emptying love.  The fact that it is a far more challenging line than Deuteronomy 24:1 misses the point.  The point is direction.  It is about turning in the direction of self-giving love.  It is about metanoia.

This change in direction is one of infinite possibility, capable of taking any lived circumstance and transforming it.  Joy and freedom flow from submission to the call of this changed direction.  Mercy is the handmaiden of such joy.  It is no accident that Francis speaks so often of joy, lest law stunt growth toward metanoia.  It is the dead community that has little faith, and relies instead upon the law.