@GordonSumerling saving the rasters as 32 bit TIFFs resolved the inaccessible files issue. Thank you!
However, the Data Upload in Create Hosted Imagery Layer is still failing. Do you have any idea why this could be? I don't think it's a licensing problem (I have license for ArcGIS Image for ArcGIS Online and my role allows me to publish hosted dynamic imagery layers, hosted tiled imagery layers, and hosted tile layers.
Create Hosted Imagery Layer Data Upload = Failed
The new Analyze Mosaic Dataset results: