Below is an example of the auto-generated update query, with client-supplied keys (_iid). There's a fatal error when _iid is not numeric. However; this should accept any value.
Question: How could I do something that would allow _iid to be more than just an INT?
You have a fatal error because the query you provided is malformed. Send something that works, and provokes the relevant error, and we might be able to help.