Which $destroy event should a directive with scope: false listen on for reliable listener cleanup?

Context

A directive using scope: false shares its parent scope and registers listeners via scope.$on(event, handler). The AngularJS documentation states that the $destroy event fires on the scope being destroyed, but the timing and target scope differ between AngularJS 1.2–1.4 and 1.5+.

Goal

Determine the correct scope on which to register the $destroy cleanup handler so that the deregistration function returned by scope.$on() is called exactly once when the directive's element is removed, without leaking the parent scope or missing the event in any supported version.

Constraints

• The directive cannot request an isolate or child scope (scope: false is required).
• The codebase must support AngularJS 1.4 through 1.8.
• No component controller or $onDestroy hook is available because the directive uses a link function only.

Questions

1. Should the cleanup handler be registered on the shared parent scope (scope.$on('$destroy', cleanup)) or on the directive's own scope reference?
2. Does the $destroy event fire on the parent scope before or after the directive's link function has a chance to run its cleanup in AngularJS 1.4 versus 1.6?
3. Is there a version-agnostic pattern that guarantees the deregistration function executes without retaining a reference to the destroyed scope?