Missing Rim Board and Exterior Wall Layers on Parapet Wall Section

[AlineEngineering]

I'm building a rooftop balcony on top of a standard garage and adjacent to a 12' tall RV garage. The RV garage has a flat roof with parapet wall surround, and the rooftop balcony has something similar but with an 18" railing. The parapet walls are built as railing walls to establish partial height. I've also set the floor of the balcony to be 4" below the floor of the adjacent room while maintaining a constant ceiling height across the standard garage below. 

 

Everything has worked out visually except for one spot on the parapet wall between the two garages at the balcony level.

 

I've tried setting that section of parapet wall to balloon below the floor below, and tried setting the lower wall (interior) to balloon above the ceiling above. I'm look for advice on how to extend this section of wall down to meet the floor of the balcony. Hopefully the attached screenshots help. 

 

I'm running X15 on MacBook Pro.