A flat bottom hull trace is often required, not only for platformers. A flat hull is just a special case of an ellipsoid. You can easily make the hull a flat disk by setting min_z and max_z tight together.
Yes, but then the collision hull has no vertical depth (and still isn't perfectly flat on the bottom). We need the collision hull to have a flat bottom/top and perfectly vertical sides that match the vertical size of the model.
