1+2: I also live in a mountainous area. First, I was tempted to use full assist in 5th uphill until I realised that the battery capacity is quite low.

I then compromised on mpg and shifted to a lower gear to let the engine do more work and save the battery. I still use assist but only one segment of the bar but for a very extended distance (it goes about 6 miles uphill, half of it pretty steep). I use 3rd or 4th. This empties the battery much less but still to about half. It's not about not using assist at all. I just use minimal assist but with a great impact because the torque of the electric engine helps a lot. Avoiding assist would mean going in 2nd with 5k rpm.

The next 6 miles are +- flat where I avoid background charge as it goes 6 miles downhill after that where I charge/coast.
It's all about distributing the load on the whole distance. The more I used the pack going uphill the more I have to charge it downhill and the less I can coast with FAS/MIMA because I have to charge more.
Not using full assist going uphill sounds like wasting gas but you will be rewarded going downhill. I still get up to 76 mpg on that road. That's not much more than usual. Ok, I don't drive with A/C.
Save your full assist to the steepest parts and downshift if you have to accelerate. Your mpg will go up but it's only for a short distance.