Hilly terrain is going to be a problem, my advice is not to try to hold a steady speed on the up hills, but let the speed drop at least a bit. These hills would impact the mpg of a non-hybrid, too, of course.

I would have a dealer look at the dieseling.

My experience with our FEH is that you have to use a light touch on the accelerator to keep it in EV mode. It takes practice and patience but can be done.

Be sure the AC is set to an 'econ' mode or just switched off whenever possible. With the compressor running, you mpg can go down considerably.

It's doubtful that your car's computer has 'learned' from the previous owner's driving habits.

Our FEH has delivered 32.2mpg over the past couple of weeks and that includes a 300 mile road trip at highway speeds and the AC on but in econ mode. Others on here report much higher readings. Your mileage, as they say, may vary.