With respect to the 42mph thing, you have to keep in mind that battery capacity and EV performance are both limited. It sounds like you maybe making a very common mistake amongst the HSD-driving public, which is to drive on electric power alone at every opportunity. Is that correct? If that's the case then you should be able to gain some measurable fuel efficiency by learning to use EV only when apropriate.

The reason this is bad is the fact that heavy travelling under electric-only uses more electrical energy than can be obtained from regenerative braking alone. That means that you might go a mile or three depending on conditions and then the ICE has to kick-in to replenish the charge. This is not a good thing because of the conversion losses incurred when energy takes that long, complicated route from the engine to the wheels. Some energy is lost to heat at every step along the way. The solution is to go ahead and use the ICE while accelerating and then allow it to shut off while coasting and drawing little or no current from the pack. As you get some charge back from regenerative braking you can feel free to use that electrical energy as needed without paying as much of a penalty, since it would have been wasted to friction, anyway. Just bear in mind that braking, even if regenerative, will still hurt efficiency. Momentum should be maintained whenever possible, and energy regenerated when you have no choice.

Seem counterintuitive? Many will agree. But you will find times when EV mode is the way to go, such as while waiting for a short traffic jam to open up or while cruising a short distance through the parking lot to your space of choice. Those are low-speed situations where the ICE uses too much fuel to justify spinning it up. But they also involve a limited duration so as not to draw too much power from the battery.

Driving in this manner should also maximize the health of your battery, although it seems to me that Toyota wouldn't let you drive in EV for longer if it actually hurt anything.

I hope this helps!