We see Mxy create a portal that leads him to the Sixth Dimension.....so they are all in the same dimension.
none of what you said made any sense. Traveling Light Hours is a measure of speed, which has absolutely nothing to do with how far he had to travel, a distance you can't quantify at all, since it was never given. He was moving faster than light, and given the battle that was raging while he was moving it took him somewhere in the neighborhood of a few minutes to get out of the galaxy he was held in, give or take.
You're right in that writers can't be trusted to be the sole arbiters of physics and whatnot in their universes, they're here to entertain us, not to rationalize every bit of the worlds they create superhero comics are fantasy fun for a reason, because the stories they tell are far beyond the realm of reality so while we can forgive them for this mistake, to take it at face value (which is exactly what we HAVE to do when trying to quantify this as a feat)), this feat is a TRASH SHOWING for Superman. 😂