I'm starting to see that reflexive acceptance of the assumption that such a definition is in any way related to the world we find ourselves in might be contributing significantly to lack of progress in understanding the bigger picture.

At any moment, our cosmos can be seen as bound by various event horizons: the Big Bang at the Hubble radius and many black holes within. Those event horizons are all very porous, but each in one direction, the CMB flux entering from (near) the Big Bang horizon and anything that strays too close exiting through the black hole horizons.

In 'Three Roads to Quantum Gravity', Lee Smolin argues that we can't know anything beyond these event horizons, then goes on to propose an evolutionary scenario connecting the two (a scenario I have some sympathy for but which I still doubt is yet sufficient to give us a full explanation for conservative physics by natural means). But the inference relevant here from Smolin's first point is that we should not use fact of the Big Bang to jump to a conclusion that even the complete "Universe" (for want of a better word) produced by the Big Bang is even bound, let alone Closed or Isolated in the sense Sung uses for clearly confined systems.

I don't want to take this any further for now. It is just that I have developed an allergy to the reflexive assumption that local truths can be safely applied to global considerations. There is not even a way to get evidence as to whether they can or cannot.