I'm not an expert on CNC machining but I don't think the keyhole would be a problem unless the corner radii are very critical . Rather, won't the minima on milling internal dimensions including angles, be controlled by practical machining criteria like the diameter and lengths of the cutters, more than the tool paths?
I recall seeing a drawing at work of something resembling the internal profile of a hollowed stepped pulley, in which one of the steps was shown with a small pocket, perhaps for a form of key, milled into it. The pocket's size and 2mm corner radii were so small, and it was so far down the overall cavity, I could not imagine how it could be machined with any standard slot-drills.