An objectively reasonable mistake for the use of self defense can be pleaded. You cannot use deadly force for self defense, when you are not threatened with deadly force. The defendant can only use force when person being defended was privileged to use force. The test is whether the person being defended reasonably believed force was necessary. Slight force is unreasonable if a verbal request would do.
Reasonable mistake is no defense.
If the person whom the property was defended against had a right to the property, there is no defense. This is an incomplete privilege and the defendant must pay damages.