I think I might have figured out what kept causing KDP to hang up when I would re-upload my book file after making minor changes/corrections.
A couple of times I've uploaded my book only to preview it with the KDP's online previewer and find an error. After fixing the problem I would upload the revised file and the upload wheel would just spin and spin and spin.... eventually giving me an error message that the file couldn't be uploaded and to try again.
I tried something different the last time I ran into this. Before re-uploading a corrected file, I saved my page to 'draft' status. Then I uploaded the new file and had no problems at all.
I'm not sure that's the absolute solution since I've only had the opportunity to try it once. But it's definitely the way I'll do it next time. (Because unfortunately there will be a next time...I'm far from perfect!)