A: Even if you set hex as alt key it will still be nullable. During the duplicate detection process, if a field has an empty value (translated to NULL in the database) in one of the fields, the record will not be identified as a duplicate. You will need the field to be business required.
C: With alternate keys you can now define a column in a Dataverse table to correspond to a unique identifier (or unique combination of columns) used by the external data store. This alternate key can be used to uniquely identify a record in Dataverse in place of the primary key.
You must be able to define which columns represent a unique identity for your records. Once you identify the columns that are unique to the table, you can declare them as alternate keys through the customization user interface (UI) or in the code.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/data-platform/create-edit-field-portal
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/define-alternate-keys-entity