We have added support for keeping default MariaDB socket authentication but we forgot to update the message in config check. It will be fix in the next minor Virtualmin release.
You shouldn’t have set MariaDB root password really. But if you did, you should have done it using Webmin / Servers ⇾ MariaDB Database Server module. Because, if you set the MariaDB root password outside of Webmin, the module won’t simply know how to connect to the database.
A user won’t be able to authenticate in phpMyAdmin with root user if MariaDB socket authentication enabled!
It isn’t a new option. We already have it. We just need to say instead of Leave unchanged, something like Leave Unix socket authentication enabled (and probably not recommend it).
Sorry, but there is no such option in the latest version of the installation. In previous versions, there was such an option. But not the last one. I noticed it right away. That is, you cannot set the MariaDB password during installation.
As for phpMyAdmin, I installed it separately from the phpMyAdmin website.