Hi @allstarsft
As for the question about replacing social login plugins:
Social login plugins generally create links between the WordPress accounts and social media accounts in their own specific way.
So nothing guarantees that the user that was registered with “Plugin A” will also be able to login with “Plugin B”
But one common thing in most social login plugins is that, they usually have some sort of email matching feature. So if there is a WordPress account with the same email address as the social media email address, then they automatically match and link these accounts to each other.
So in most cases people will still be able to login, but there are providers that give people an option to hide their email addresses. In those cases the registered WordPress account can have a completely different email address than the social media email address, so this email matching can not work.
Anyways Nextend Social Login can display link and unlink buttons for the logged in users, so in these particular cases if the person can login with the traditional username / password login, then they could link any social media account to the currently used WordPress account ( if the social media account is not linked to another account already ).
As for the question about the unlinking of the users:
If those users weren’t linked by the Facebook provider of Nextend Social Login then they don’t need to unlink their Facebook accounts, as we won’t know anything about the links created by your previous social login plugin anyways.
The unlinking process that our documentation mentions is only necessary if you replace the Facebook App in the social login plugin that your users used to link heir Facebook accounts to their WordPress accounts.
As for the?next best step forward ( original verification problem):
I would suggest continuing with the plugin/theme conflict test as I described above, as that way you could figure out if the problem is caused by a third party or not.