Barring something like a city decimating earthquake - for every foot you ascend, you must also descend. There is just as much uphill as downhill. This is true for any trip with any two destinations on the planet (a round trip between a beginning and a destination). This is equivalent to conservation of potential energy in physics.
There are a lot of mountains in the US where you're going to spend hours driving up steep inclines. I used to live on the wrong end of a 2 hour uphill drive with one gas station in the middle. That road was steep enough that even a gasoline car couldn't make the trip without a full tank of gas.
I drive through mountains all the time, and have to fill up my car all the time. GP didn't say anything about multiple stops; what I inferred (because it's literally what he said) was that you can have unequal ascending and descending on a round trip - which is geometrically impossible! And even if we are talking about having to stop and get gas, you still get the same benefit of regenerative braking when you are descending, whether it be the first half or the second half of the trip.
Have you never been up a mountain? You can go up one very steep portion at 45 degree incline and then come out the other side going on a very long down hill at like 5 degree decline, basically flat.
It doesn't matter. The battery is still regenerating on the way down until you're back at the start point. A less steep angle might even be better because there's no potential to lose energy since the battery can only regen so much at a given time.
Not if the mountain's big enough. You might only go an appreciable distance downhill on the way home.