Is a wife's refusal to cover her hair in front of men who might pass by, at her mother's request and while being filmed, considered a form of defiance (nushuz) that warrants abandonment or physical discipline? And does the matter escalate to considering divorce?

The wife's behavior does not indicate corruption or defiance. It might be her own interpretation, as she might have thought that the men would be preoccupied with food after her mother called her. Do not let satanic whispers drive a wedge between you, for separating spouses is one of the devil's greatest temptations. Preserve your family and raise it upon goodness and faith. Think well of your mother's insistence that your wife go out to greet the guests, for Allah Almighty says: (O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin).