Sweet Home 3D doesn't fully support OBJ format, and Tf is simply ignored in the current version. Anyway, I think the missing factor is light reflection, not light transmission. Your solution with a small light located within the light structure and one a outside seems a good trade-off. Using a smaller value lightSourceColor value for the small light source should be better though.
You could also try to compute an image with some light reflection, using Advanced settings of photo rendering plug-in described here. Setting globalIllumination with path value gives different results with only one source light placed at the center of the lamp but it takes much longer to compute with some noisy results.


Double light source with default renderer setting


One light source with path value for globalIllumination and 1 as diffusedBounces

By the way, when you create a sh3f file from an other one like you probably did, you should at least remove or change its id found in the PluginFurnitureCatalog.properties file, otherwise it could mislead the auto update system in Sweet Home 3D with the updates of existing libraries.