@Cerrydd First of all, I had to LOL regarding your window placement comment - you sound like me! :D
Very inventive use of stairs there, and I'm sorry it doesn't wind up looking exactly as you might have liked. I don't get why they are so poorly aligned.
Regarding your lighting issue, did you by any chance try rotating the home on the lot to see if anything changes depending upon which way it faces? That might help us figure out if it's being affected by the ambient "night" light. It sounds like you did observe an effect from interior light. Have you tried plopping a sim on the lot, turning off all lights, and then only turning on the exterior ones? That would help to confirm if it's an issue of interior light affecting exterior (beyond what it should).
That trim has always bugged me too, and I sort of had it lumped into the ceiling recolor request, but I think stating it as a specific request couldn't hurt!