Both. Both is right

Go to Render Settings>Light Paths>Transparency and turn that value up till you can no longer see the black show up.

The Curve Minor is the shape of the actual aroura. it might look flat but it's actually not. Though it's barely noticeable.

Yes, there are two methods:

1: just use an area light and 'fake' the light.

2: If you're using cycles you can connect a Mix Shader node to the Emission shader in the graph, then connect that Mix Factor from a Light-Path Node 'Is Camera Ray'. and plug the remaining input to another Emissive Shader, and then you can edit that. Here's an example:

This works with the newest Version of blender (3.3) and should continue to work for the foreseeable future.

However it might not work back in 2.8 So if you're on an old install of blender, update!