I Already Imported Everything. Can I Still Fix It?
Yes. Fix the original files, then re-import, or point the app at the folder it imported from.
Yes, as long as you still have the files. Most photo apps copy them into a library of their own, so the originals are usually still sitting in the download folder.
Fix those originals, then remove the bad import and import again. It feels like starting over and it takes minutes rather than the hours the first import took.
Takeout JSON Metadata Fixer writes a clean, dated folder you can import in one go. It runs on your own computer and is free for your first 500 photos.