I think your constructions are totally impressive!

The Dining room, taken from the corner near the doors to the back porch. I don't know why it came out so dark... maybe I need to adjust the luminosity of the hanging lamp...

A stronger light source will just create an even more contrasty rendering. To lighten the room, you need to think like a photographer.
My advise:
As a general rule you should never use the "Add ceiling lights". Instead you should place some light sources around the room, f.i. use five - six incandescent light sources at around 5% strength and increased size, to create focus on the most important details.

II have posted a thread about lighting HERE
You might find something of interest there.

cec