Can I undo an import if I'm not satisfied with the results or if it causes issues with my account?

Simkl provides an option to clean your watch history to zero using https://simkl.com/settings/login/clean-or-delete/

To be on the safe side make sure to create backups using https://simkl.com/apps/backup/ before importing any data. So you can restart as if nothing happened if you mess up the import.