I think your plan sounds good for the most part, though I would probably stick to C-hops for the late additions to get grapefruit aroma/flavor from another angle. Also, I would just make a tincture with vodka with the actual grapefruit zest which you can add at packaging. Maybe add a small amount of carafoam to boost head retention in light of the vodka, but 20% wheat should give you plenty of dextrins in there.
