It is not possible with any process- not with data loader, process builder or apex. You can use apex to copy the attachment with the other parentId and then delete the original attachment.

Using @isTest(SeeAllData=true) is baa practice, as data may be different in each environment, and therefore the test behavior will be different.

 

You should create all the require data in the test class. Custom Setting data can be created as any other Object records

No. You cannot.

In workflow you need to user either email field or user field type.

You should either use code that extract the emails from the custom field, or at first store them in separated emails fields without comma.