Some of these hill are quite long and steep.

If I keep it in D I have a couple of options:
1. Let the foot completely off the gas and let it idle. However if I do this there are a couple of additional factors:
a. IMA will kick in 4 or more bars of recharge, slowing the car down.
This is great if you need it, but not if you don't.
b. Along with drag from the IMA, you're also being limited by the drag of the friction caused by the spinning ICE.

2. Apply enough gas to turn off the IMA recharge, and to overcome the friction caused in the spinning ICE.
This is fine, but as pointed out you must add gas in this case.
The gas that is used to overcome these factors are burned, and not saved in the tank.

The most reliable method of gauging FE is the FCD.
Sometimes I'll keep it in gear and apply #2 above and can see the instant gauge move slowly up to the 120MPG mark and the average increments very slowly.

Alternately, I can switch to N and the instant jumps right to 120 and seems like it would go quite far if not the limitation, but more important is the average, which is incrementing at a much faster rate than if left in gear.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT