I would make the bottom of the Profile start at the bottom of where the two red skins join.
Did you decrease the height of the Wall so that the top of the Wall is at the bottom of the Beam Profile? Then the skin fills should merge with no lines.
Same goes for the Roof: the vertical left edge of the Roof should coincide with the right vertical edge of the Beam Profile. Everything left of the Roof and above the Wall should be profiled, including that little overhanging part of the Roof.
As far as I know, there is no such developments in priority based connections in AC18.
One thing to enhance L-joints is a new checkbox in the Wall Settings Dialog called End Surfaces: Override using Adjoining Walls. If it is checked the edge of the Wall will use the surface of the Adjoining Wall, even if it is the stronger Building Material. Check the attached image for explanation.
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