I still have the catalogues and a heap of LRPREV files. Probably half the images in the catalogue don't appear in LR with a preview.
This may be a stupid question, but are the images showing as being present in the catalogue in the grid view, but just without a preview image? Have you tried simply generating new previews?
Another possibility, and I don't know how to get around this one - if you open LR and ALL of your images in the Library grid show up as empty gray boxes, it's usually an indicator that your monitor profile is corrupt and you need to re-profile your monitor. I've never seen that problem with only part of them before. You mention that probably half the images don't appear in LR with a preview. Are they all one file type - JPG or RAW or DNG or some combination thereof?
By default, LR doesn't right out any of the metadata in its catalogue to the image files/sidecar files, but you can force it to do so by selecting images and pressing Ctrl/Cmd-S. What happens if you write out the metadata to some of the files that aren't showing with a preview, then create a new catalogue and import those images you've just updated into the new catalogue? Being 'new' files for THAT catalogue, LR would generate new previews for them. If you've written out the metadata then LR should import any Develop settings, keywords, etc. The only reason to use LRViewer and programs of that ilk is when the actual image files themselves have gone missing but that's not what you're describing.