So the grandma eventually responded your FB message?
Honestly my philosophy has always been getting a cheapest new vehicle if possible. There're too many uncertainties buying a used car and you just never know. Besides, paying $15,600 for a 2014 CX-5 Touring AWD with 70,000+ miles? I just don't know. I remember a guy here paid $21,000 with incentives for a brand new 2017 CX-5 Sport a while ago, although he had to drive 500 miles to get it. That'd be something I'd try to get if I really want a lower priced CX-5.
If you can have this used CX-5 checked, make sure to check cylinder compression and that can tell you much of the engine condition. And no there's no way that the oil can last 30,000 miles without changing. The oil will become sludage long befor that and the engine would die.
I found their phone # and talked to the guy for awhile. He also gave me the name of the shop that had been doing recent service so I called them up too and verified several oil changes were performed.
My mistake may have been telling the dealer I talked to the former owner, because now he knows I'm aware it's in good condition. OTOH he knows I'm aware of what they traded it for.