Thanks. I will uninstall the plugin, on the basis that if the situation is not improved next time for native merge process, I can always presumably reinstall it.
I wonder if the age or source of the posts being merged was a factor? These posts were all imported from a now defunct Yahoo Groups forum, and the import process created ‘zombie’ users where there was no corresponding active user for the given email address already. I think this old user account was an example of this.