There is no such thing as “read-only” in physics. If we can observe the other galaxy, that means it is affecting the particles in this galaxy. Even if it’s just a single molecule in a telescope responding to a photon that hit it from the other universe.

I guess that makes sense, thanks. I was wondering if it might have been something like the theories of what might happen much later in the universe's life, where things eventually get so far apart while still accelerating that light can't travel between them. But it couldn't be something like that if we can see the other galaxy, right?

We can see light that was emitted from it billions of years ago, before we split apart. Similarly, they can see light that ours emitted billions of years ago.

They can never be affected by anything we do now, or vice versa. This doesn't make it "read-only", though -- we can't read it either. All we're seeing is their distant past, back when we were mutually able to reach each other.

At some point in the far distant future the night sky will be black, with everything being beyond our visible edge of the universe.

That still holds true, eventually nearby clusters will go beyond our "observable" view.

True, but we can see galaxies that are already over the "light horizon." In other words, we can see light emitted from those galaxies when they were within the light horizon (the point where they were receding from us at less than c), and that light has a causal effect on us. Light emitted from the galaxy now, however, will never reach us because it is already receding from us faster than the speed of light.