Duplicated scenes multiply objects, datablocks and file clutter, and they drift apart over time. Setup Switcher keeps one scene and stores the variations beside it. You can also load just part of a setup, which a duplicated scene cannot do.

Loading replaces the saved categories with the stored values. It is an undo step, so Ctrl + Z brings your previous state back. For extra safety, save your current state as a setup before you load another one.

Yes. Expand the setup, click Load Selected Parts, and tick only Lighting, World and Color Management. Everything else stays as it is.

Setups that store geometry and packed textures add data to your file. To keep it lean, open the gear settings and turn off Geometry, or turn off Pack Texture Copies, when you do not need them. Saving a setup with only lighting and camera adds very little.

When you load a setup with Visibility enabled, those new objects are hidden in the viewport and render, so the scene matches the saved state. They are not deleted. Objects you deleted after saving are not brought back.