20+ years in law enforcement speaking here...
First of all, echoing all above who say to pursue criminal charges. Get the names of all possible witnesses and get statements from them ASAP. Some of the best silent witnesses we have these days are cameras on things like ATMs, stoplights, small businesses, etc.
I would be most worried about the photos. Why did he take them? That is not a normal course of action for an angry person who is themselves doing something wrong (threatening). I would be extremely concerned that he plans on running MIL's license plate and tracking her down. Not sure about PA, but in some states license plates are viewed as the property of the state, and the records behind them (owner's name, address, etc.) are available to anyone, especially if the incident occurred on a public thoroughfare.
At one of the first agencies I worked at, small town PD, we had a similar-sounding guy who would come to the station weekly to get license plate records for any car whom he perceived as using his driveway as a turnaround. Trespassing and disturbing him, as he saw it. He said he was just "keeping them on file" in case he had a break (large property w/outbuildings). He had a screw loose, but it took a while for anyone to be concerned. Eventually the requests got so numerous that the Chief told him to write to the state if he wanted more records.