As far as I can remember there is no option for not charging the car on ICE. its only low, medium or high. Yes you will use slightly more fuel when charging the car using ICE so I only ever have it on low. I normally save the EV for when only city driving as I find EV is most efficient at lower speeds.
At 30-40mph I switch to ICE and let the car decide what is best. If you let the battery run down completely and you still have a long way to go, the fuel economy drops considerably as you are at that point using a hybrid as just a petrol car but with the additional weight of a battery.
Save the EV for when the miles left is similar to your destination so you can charge when you get back. I have done a comprehensive review on the PHEV and EV in general you may want to check it out