Thank you for all this info, Owen. I can now successfully load a single KML as a file to my ArcGIS Content, and open it via Map Viewer, then save that map for use within a StoryMaps feature.
However, I cannot load multiple KML files to use, for example as multiple layers, because Map Viewer still technically limits file inputs to only Shapefiles, GeoJSON, GPX, and TXTs (although I don't know why - obviously it can visualize KMLs as well). Because the KML file size limit is 10MB, I can't load the KML I need to use (it is closer to 40MB). I also cannot break the KML data into multiple KMLs displayed on a map, because of only being able to open the KMLs individually by navigating via the ArcGIS Content list.
Is there any way to add multiple KMLs to a map or to use a KML over 10MB?