Since a new library does not have the "Last Import", it is clear, that your current library is damaged.
Photos does only have the one repair tool, that you already tried without success.
Can you drag the Last Import from the Library section to the "My Albums" section? If you can get it to "my Albums" you may be able to delete it.
If all that fails, you would need to do "something drastic" as Keith earlier wrote. Or wait for a system update, that will recreate the library.
You did not yet answer my earlier question about iCloud Photos.
If you are using iCloud Photos, the best way to create a clean new version of your Photos Library would be to enable your new, empty Photos Library as your System Photo Library and to sync this library with iCloud. All photos, albums, metadata will download from iCloud to the new library, hopefully without the errant "Last Import".