Hi Drew,
The end trim uses the same material that’s selected for the internal side of the wall, so if you switch your “none” & 1/2" lining materials & draw the wall in the opposite direction (so the external side is along the other wall) the end trim should be correct.
Note: The 3/16" trim won’t have a material applied & the thickness is just a backup size.
Tip: To save time when this happens, rotate the wall using the SketchUp Move Tool (so it rotates around itself) & then edit the wall with the first part mentioned above.