Your fraud scenario is just straight up not going to happen. It's significantly cheaper to run on electricity instead of fuel so it's in the drivers best interest to charge as often as possible.
Except that it does happen a lot of time. According to Volvo's own data 60% of PHEV buyers never plug them in. I have a guy on my street who has a Cayenne E-Hybrid and I've never ever seen him charge that car. Yeah it's dumb, but it lets you buy a very expensive car and deduct almost all tax if it's a company purchase.
