hi,
Can we change, the group: account_credentials (alias: account_credentials to default) and convert to CORE; then change group: user_information (alias: default to user_information) ?
Sorry this is not possible, but you can use Batch (see buttons in the toolbar) actions to move fields from a fieldgroup to another.
No to do all the work again, we need that after user entered all non core information, they can't make changes, changes will be made to profile through request to our attention center.
This is not possible, keep in mind that Easy Profile extends Joomla user management, not replace, so fields like username, name, email.. are required by Joomla. If you do not want to allow to change some field then you need to use Readonly parameter, but these fields need to be available in Registration and edit profile fields.