A quick update on my last post here. I may have stumbled into the problem, at least on my system. It's ComicTagger. So what I did was copy a CBZ file from my external HDD where my collection resides to the Desktop. Still a blank icon. I Force Quit Finder and re-launched it. Still blank. I then decided to start removing apps related to comics on the thought that there was a conflict, and put ComicTagger in the Trash. Then Force Quit Finder, re-launched. And BAM! The CBZ file on my Desktop now has a thumbnail!
Now, my archive on the external HDD are not generating thumbnails, at least not yet. There will be a lot to get generated. But QuickLook is working.
EDIT: thumbnails will generate on the HDD if I adjust the size of the icons up some. This is actually a problem with EdgeView's thumbnail generator, which only does them when Finder is set to show icons. But disabling EdgeView's QuickLook extension in the System Preferences/Settings fixes this and lets Simple Comic's extension take over.
EDIT 2: I wasn't crazy about leaving ComicTagger off my system entirely, but i couldn't identify what component in the package was causing the problem. So I moved the app to my User Applications folder and thumbnails continue to be fine.