Layout plan and sketchs look great.
Thought you might be interested to see this pic which I took at the Cheltenham exhibition a few weeks ago, I know its not 15" (On16.5) but the setting is similar with dimable lighting and lights in the offices . I was really impressed with the 'staging' of the layout particularly the way the buildings formed a 'set' around the sides and rear of the baseboard with no need for the (all to often unrealistic) backscene. It did however seem a shame to go to the trouble of containing the scene within a proscenium (which is great) only to spoil the effect by sticking a smoking chimney on top of the box! The other thing for me was the need for alot more grime, but it sounds like you are planning plenty of that!!
Looking forward to seeing more of your layout.