Kmak:
You are correct. As the z height changes, the laser will need to be refocused. It may not be an issue for small prints, but for larger prints, focus could be a problem. However, if you float the inside container inside the outer container, and attach the peachy to it, the laser optics never need to be adjusted. See here (design by Spelljammer, here on the forum.)

The reflecting mirrors do affect the power of the laser, but not enough to be a problem. The resin only needs a few miliwatts to cure.