That's in a nutshell what I am asking.

The Law of Conservation of Matter and Energy suggests that E + mc^2 should remain constant over time. However if new stuff is being added to the universe because we are inside a black hole, E + mc^2 should be a function of time and not a constant value.

It seems there is a testable hypothesis for someone.

If it is true that stuff is sucked inside our universe from the parent universe, I imagine the same must be true for "our" black holes: they suck matter from our universe to the child universe inside them...

And having the sum of stuff sucked in and sucked out to be zero, there should be a limit to the number (or dimension) of black holes our universe can have (?)

But the mass of the black holes from the outside universe count on what I call the energy/mass balance sheet, don't they? In other words, for the parent universe I would think you get to count all mass and all energy in all child universes. But in a child universe I don't see a way to count the parent's mass and energy.

What if there's a symmetric relationship - I.e. our "parent" looks to us like just another black hole that we, not knowing better, would consider a "child" of our universe.

Hawking radiation in one is from stuff falling into the black hole from the other.

Then everyone is on everyone's balance sheet, and e+mc^2 = constant.

Topologically it would look pretty crazy.