It seems you're using an old version of Blender (like 3.3 or older), where the Mix node was different. Its not possible to open shader groups created with modern Blender versions in older versions without having to refactor the whole node graph.

In Eevee, the viewport can slow down when you move very close to the planet. That is because the number of node graph evaluations increases drastically when the planet fills most of the viewport. For a snappier viewport, you can switch to Cycles instead. Render times in both engines will still be quick compared to conventional path traced volumetrics.

That's fine, to "illuminate" the planet only using its internal emission data, you can increase the Brilliance slider to 1 and set an Empty object as the "sun" object in the Geometry Nodes panel (or you use the existing sun object and disable it from showing up in the viewport and the render).

When you work in Cycles, you can also make use of light groups to treat the planet and your foreground separately.

Make sure that your sun lamp is selected in the Geometry Nodes Modifiers tab of the planet. This is needed to calculate the directional vector between sun and planet. Also make sure that there are no other lamps present in your scene.

The compositor effects included in the original file are meant as a starting point or as reference for your own compositor setup. You can either use the original blend file directly to create your renders (the easiest way to access the compositor nodes), or you import the glare and flare node groups by appending them to your project.