Although I'm curious and love a challenge, I'm going to stop trying sideloading. It never works for me, and for some reason I now have 2 or 3 books (the ones I was messing around with sideloading) that won't show covers even when I get them from the personal server. I see no evidence of the books on my PW - nothing in the thumbnails folder, etc., but somehow repeatedly trying to sideload has made my PW cranky towards those books.
Downloading from my Calibre personal server, after following the steps above, always works for me, as long as I haven't experimented with sideloading first. Makes no sense, I know...