I assumed the sync provider hints the Contacts app on which default fields to show.
The sync provider can indeed tell the Contacts app which fields it does sync, so that the Contacts app doesn’t even show unsynced fields (extreme case: read-only sync adapters like a so-called Facebook which provide no fields to edit).
However, it doesn’t provide default values and has no influence on further Contacts app behaviour.
Maybe you can find a contacts editor that better matches your needs There are different ones out there.