The elevation is still off. This might be caused by unsupported tags in the KML file. If you open the asset in Sandcastle, you can see in the console an error saying “Placemark Regions are unsupported”. CesiumJS currently does not support Placemark tags. You can read more about KML support and roadmap here: KML Support · Issue #873 · CesiumGS/cesium · GitHub.
I am still looking into the GeoJSON file. I will get back to you soon.