I can’t read your R7 .3dm file in R6 so started with @HS_Kim’s code that has your geometry internalized.
I manually selected two of your breps and stretched them in the Y direction so they won’t fail to RUnion, but there are other poorly sized walls that also fail. I haven’t got time for the pain (Carly Simon). Oh, and I divided area values by 100 because they are so large (millimeters?).
getArea_2021_Aug5a.gh (70.2 KB)