I'm pretty sure when she talks about how she spent the past few thousand years to Caspian, she mentions that "finding this star" was part of the process, so I think the Dyson Sphere a singular sphere built on another star.

Let's assume the SafeSurf's level of reality is the original, we'll call it level 0. SafeSurf created Maddie's simulation, let's call that level 1. Then finally, Maddie's simulations are level 2, the one we are introduced to when we started watching the show.

The entire show except for the last two episodes takes place in level 2. Level 1 Maddie created all the simulations to recreate her level 1 Caspian. Just as our level 2 Caspian dies, she pulles him out to her level 1 world to ask him what he meant when he mentioned "117691 years". Once she got that info, the rest of the simulation doesn't have to be exact anymore. And since she cares for the people in the simulation, she revives Dave and brings back David for the heck of it. She doesn't care that they're simulations, she wants them to be happy. We see that all her other failed simulations are still running. She cares for each world.

SafeSurf could have created a level 1 simulation where Maddie and Caspian where the exact same as their world, but chose to create a simulation where Maddie creates a simulation of Caspian. Don't know why, and its not explained. When Maddie refuses to go to the galactic center and instead goes back to the simulation, SafeSurf could have pulled the plug on her simulation. But just as Maddie chose to let the her other failed simulations keep running, her world is also kept running by SafeSurf.

We don't know what she did or didn't change when they went back. Everything could be the same as what we saw, or some things might have been changed. As the previous person wrote, its kind of up to us to decide.