A few notes:
Overlapping parts and adjacent parts don't refer to subroot indices. They refer to list indices (since more than one part could reference the same subroot). So double check that.
Overlapping parts and adjacent parts are not automatically reciprocal. So if A is adjacent to B, you also need to tune B to be adjacent to A, if that's the behavior you want.
Sims place footprints when sitting down, and that might be blocking slots when using the corner seat. You might have to "stretch" the L connector so that's not the case. From the screenshot, it doesn't look like it should matter, but it's hard to tell since you don't have access to debug visualization.
