One way I have resolved a similar issue is to assign a random username and random strong password to the users during the process of creating a new user. You don't need to show these fields on your form, just ensure that they have values. Community Commons module has Java actions that will create random strings and strong passwords.
BTW, If you haven't already, you'll want to use the microflow Administration.NewAccount as a starting point for creating new users. I copied that microflow into one of my modules and then made modifications to it.
Hope that helps,
The errors you get are because of the Validation Rules on the entity User. You cannot modify these validation rules because User is a system entity. I don't know the use case but maybe you could look at the Anonymous user tab on the Project Security.