But you can't do that for the universe as a whole. Asking what would be the case if you could is meaningless; it's like asking what would be the case if 2 + 2 were 5. No consistent model exists of such a situation, so the question is meaningless.
> if the initial thought experiment was posed as 'observable universe' would it make a difference to you?
It would address my "can't operate on it from the outside" objection, yes. But it still wouldn't make the thought experiment meaningful, for the reasons I gave in response to wyager elsewhere in the thread.
Original thought experiment said matter you're talking about total energy in your response. We ought to take matter to mean ordinary every day matter, mass being the property that all regular ordinary matter posses. Since that's normal everyday language.
Yes in a closed universe the net energy is 0, nothing in; nothing out, so you wouldn't be able to magically 'pop' mass into one place without taking from somewhere else.
Now if mass A is 100 meters away from mass B it would take X amount of energy to push/pull them together. If they are now 150 meters apart now the energy required to bring them together is now higher.
So if galaxies are pulling away and away from each other over time then overtime the energy you would need to bring them closer together again increases.