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Create a new catalog, put one or two photos in there.
Then in the new catalog, try File->Import from Another Catalog and point to the main catalog. See if this allows you to work properly on all the photos and make proper backups.
Check your hard disk for bad sectors and other problems, there's a good chance that might be the cause of all of this.
Take one of the backups (assuming it was on a different disk) and try opening it in Lightroom on a different computer.
Zip up the .lrcat file and put the zip in Dropbox and PM the link. I can take a look at it and see if it's recoverable.
I've tried your solution but LR crashes when importing pictures from the old catalog: "Lightroom encountered a problem while reading etc etc".
I am doing this from my second machine, so this does not seem to come from the disk, but from the file itself.