I have successfully installed it and run the command:
php bin/magento ok:urlrewrites:regenerate
However, when I try and add products to a category I still get one of the same errors as before. I am thinking it must still have original re-writes held somewhere, otherwise it would not have known about them? Perhaps I should truncate as suggested by elfling.
For anyone else.
Truncate url_rewrite and clear all the 301 redirects. This will clear the issue and you can regenerate all the URL's with the module and you'll have no further issues.
Thank you for the information. I am running into this same problem and will probably use this module. Where do were clear the 301 redirects from?