Yes, there is a “bug” currently. We are aware and will add MariaDB 10.3+ in the future.
You can always change the password for your db user using command line.
Concerning your first question, if you add all databases to be managed by this virtual-server upon phpMyAdmin, it will work now. It’s just that a user password will not be updated, so far.
I have seen how to create a database, but I am unable, to add edit user name for that database. Doesn’t this functionality exist in this panel?
Database takes the same username that is used to log in.