The indication and behavior around partially selected branch indicator with all leaves selected is what sold me in particular. Agreed that multiple visualizations could work and that getting the behavior right is the crux of this problem. My question was perhaps overly lengthy, and I agree that details about visually how selections are transitioned from unselected to selected is less important, although your basic visuals are already quite satisfactory. Your state representations and overall behaviors address all the "challenges" I listed.