Lightroom Catalog Errors/Corruption

I have a primary catalog with 500K images, and it has become all but unusable due to slow responsiveness. I have done all of the testing and optimization, and I even attempted to export and reimport the catalog into a new fresh catalog, hoping it would rebuild. Any attempt to import it en-masse into a new catalog results in an error about halfway through and 60% of the images not importing to the new catalog. A final attempt at rebuilding a copy of the catalog using SQL resulted in a massive error log, so I know the file is horribly corrupt, even though LR does not seem to think so during normal use. It has been backed up constantly via Time Machine but the corruption history goes back MUCH too far.

Can someone help me get this thing repaired? I cannot afford to lose years of collections and selection data in a complete reimport of images.