1. My suggestion to accomplish this would be to adjust the collision normal accordingly. You could have one normal at the top corner pointing, say 45 deg, up/right and one normal in the bottom corner pointing down/right. For any collision between these you can interpolate between these two normals (using Vector2.Lerp) to get an angle that will then point stright to the right in the exact middle of the edge. In case you want a region with perfect horizontal bounce you can use two normals on the borders of this region pointing in the horizontal direction. 

2. You can handle this by using different normals for different edges.

3 and 4. You are on the right track. In the future you can implement a sweep and prune algorithm to determine for which entities to make the broad phase collision detect. For your purpose there is no need for it right now as you only need to check the ball against all other entities which is a O(n) operation. If you have many moving entities which may hit each other you might need it as check all these against each other will be a O(m^2) operation (m number of moving entities that can hit each other)


EDIT: Note that when interpolating two unitvectors you are not guaranteed to get a unit vector back, it will probably be a little shorter. So make sure to normalize after interpolation.