Suspect is hired to protect 3rd person's tangible movable property after numerous thefts. Mother and son stop in front of property at night because a bottle (not beer) rolled into drivers floorboard. She sees pickup pull from behind a structure and head towards her. It scares her because they were on an untraveled country road. Suspect catches up to her after calling his employer to report what he thinks is the thief and tries to make her stop by going around her and slowing down on the 2 lane road. She tries to get around him and rolls her van three times. No SBI and no physical contact between vehicles. Do you think PC 9.42 would be a decent defense to agg asslt deadly weapon?

It all boils down to reasonableness. Was the actor (suspect, defendant) reasonable in (1) believing that the victim was, in fact, the thief, and (2) in the use of force (in terms of how much force was used and whether it was immediately necessary to prevent the theft)?

My view under the facts as stated: suspect did not act reasonably. (1) What evidence, other than the fact V slowed down, was there that V was a thief at all? (2) Even if you assume that suspect's belief (that V was the thief) was reasonable at the time V slowed down near the property, once the V drove off, the use of any force at all was not immediately necessary to protect whatever property your suspect was protecting. The use of a vehicle to "corral" her away from the scene was probably overzealous force to protect the movable, tangible property that he left behind.

I think the problem you are more likely to have is related to whether he disregarded a substantial and unjustifiable risk that she would get hurt somehow by his actions - the result here was that she rolled her van 3 times. Was this (or some other injury) a foreseeable result under the facts of your case? Speed, road conditions (e.g., dirt road vs. paved, bar ditch vs. flat land), etc. could all contribute to whether the suspect ignored a substantial and unjustifiable risk that his actions would cause injury to V. I'm not making a judgment on this issue, I'm just throwing it out there to consider.