Puzzle Template: trying to replace Directed Light with Rect Light

I am new to UE4, and after watching some basic tutorials, I opened up the Puzzle Template.

I want to simulate a 2D view, so I changed the camera to orthographic, centered it over the “area of interest”, and made sure to have it pointing straight down along the Z-axis.

When I run the template, I get most of the 2D effect I am looking for, but I can still make out some non-uniform shadowing (dark edges)/light reflection (bright edges). Turning off “cast shadows” on the light source didn’t help, so I tried setting up a rect light that is as large as the region of interest (pointed in the same direction as the camera, as large as the area of interest). The problem is that regardless of the strength of the light (in cd), My scene remains at the same level of darkness:

Any hints as to what I might be doing wrong?

Have you tried tweaking the exposure values on the camera?