> For an infinite universe or multiverse with infinite mass, you'd need infinite energy, right?

Given that the current best guess for the geometry of the universe is (conformally) flat, this universe already contains infinite energy. From what I recall, the normal definitions of total energy of a spacetime are not possible to formulate for non-asymptotically flat spacetimes. So it is a bit nonsensical to talk about the total mass/energy.

The easiest skeptical take on multiverses is simply that there are no measurables defined by multiverse theories yet. I usually lump them in with anthropic theories, which a lot of GR theorists seem to love.