You don’t need to set the Diffuse to 0% for everything, just that example of the area environment sphere. The reason I do that is so the light coming from the sphere will not light up the sphere more than it needs.
A Diffuse of Grey shouldn’t matter.
Light too close to the wall, depending on the shape of the mesh, if a plane and the light is pint toward the wall then yes that would be an issue.
Area lighting is what other software call Mesh based lighting. Providing the normals are correct any mesh can be used to emit light. You can treat the surfaces just like other surfaces. You can put a transmap in and hide parts of the mesh etc.