I thought pfSense was working on the Pi, since Gonozopancho did it, but I guess he didn't make a how-to. ;) OK, so that's out. As for ad hoc network I had even forgotten that the damned thing excisted, from back in the 90's when it was actually in use! ;) I agree that if it had been a cowboy car factory (like so many electric car companies now) it could be a problem, but Mitsu has been around for a long time, so I wouldn't be more scared about them stopping the service then Volvo doing it for my car. And malware is not really a thing with simple stuff like setting the heater, but with a Tesla I agree it could be a problem.
I will take the extender out there some time during next week and see if that picks up anything, thanks!